It's an accusation Snape made in the flashback of book 7 that Dumbledore didn't dispute. That's why it's a common refrain; it's something other characters in universe noted.
We're going to trust something that Severus Snape of all people said, in the heat of the moment and right after being given the news that the boy he'd been trying to keep alive for so many years needed to die? All because Dumbledore, a man of infamously guilty conscience, refused to dispute it?
Even being extremely uncharitable to Dumbledore, "raising Harry like a lamb for slaughter" just
can't have been the plan all along. Dumbledore explicitly didn't know anything for sure about Voldemort's horcruxes (and thus what all that Dark Magic in Harry's scar was) until he saw the Diary in book 2. Before that, raising some sacrificial champion would have been counterproductive even if he
was the typical Manipulative Old Bastard some people like to make him; it would have been a
much better use of an old man's time to raise him by hand to be a the tailor-made successor that the wizarding world expected him to be so he could carry on Dumbledore's MOB schemes after Dumbledore's inevitable death due to old age.
Be open and honest? Tell Harry what he has researched about Horcruxes? Actually talked to other magical experts about what's going on instead of keeping it all to himself? Give Harry the training and resources to, I don't know, have more than a snowball's chance in hell of succeeding?
Harry was a 16-year-old kid at the time. One with a talent for combat magic, maybe, but still a kid who had no way to measure up to Voldemort in open combat. How was Dumbledore meant to increase his chances? Get help from all the people who got steamrolled by him the first time around? Get help from an international community that was perfectly happy to sit back and watch the first time around and whose opinions likely haven't changed?
While it's nice to assume that the books have a stupendously insular view of the British Isles, (because they do) it's also never contraindicated in the text (to my knowledge) that Dumbledore could have asked for help from any and every expert in horcruxes and cursebreaking he could get his hands on. It's even, I would argue, relatively likely that he made subtle inquiries about the subject to anyone and everyone he could trust and who had a qualification related to the field
and found nothing. It might be authorial fiat that there's no way (outside of the nonsense that happened in Deathly Hallows) for the host of a horcrux to survive its destruction...but as much as I hate to say it, if Rowling decided that's how living horcruxes work then
that's just how they work. And that's not Dumbledore's fault at all. It's just a (mostly) immutable rule of magic in the setting.
Harry survived and won through a combination of events and effects that NO ONE, not even an expert in his field like Ollivander, has ever heard of. OOC, we know because it was authorial fiat, but IC, there was no possibility that it could be planned.
Which effects are those? Again, this is a genuine question. None of the ones I can think of at the moment were literally unheard of.
Unless you're referring to the blood-connection-protection flapjack that let Harry come back from the dead, but I'm not going to argue about that one being total and complete hand-of-the-author-creating-happily-ever-after. Though if you want a Watsonian explanation for Dumbledore's part in it, Dumbledore had no reason to expect it to work. He just also had no reason not to try it when Harry needed to die anyway according to literally every source he could find.
Even if we assume Dumbledore knew Voldemort would be made completely mortal by Harry's sacrifice – that he would have gotten all of Voldemort's protections before going to fight the BBEG – that only puts Voldemort back in a state he would have been before 1980.
In 1980 Voldemort had multiple horcruxes. Indeed, it's heavily implied by the way he sought out Hufflepuff's cup and Slytherin's locket as 'Tom Riddle, employee of Borgin and Burke's' that by the time he was introduced to the world as Lord Voldemort he had
at least two horcruxes, and that's not even counting the Diary and anything else I might be forgetting about.
Furthermore, while it's never explicitly stated in the text, it can be reasonbly intuited from Voldemort's physical appearance and the way no one remarks that he looks different now than before he was resurrected (not even Fudge upon disbelieving Harry's account in Goblet of Fire chooses
that as a point of contention) that Voldemort performed at least one and more likely
several dark rituals upon himself. And if he was doing that, it was almost certainly with the purpose of making himself more difficult to kill or otherwise more difficult to face in combat. There are other things he could have been doing to himself, I suppose, but for a man like Voldemort combat prowess would have been a priority.
And if there wasn't some ritual in Voldemort's body-modification set that anchored his soul a little more firmly in his physical form I would be very surprised. Voldemort isn't the first wizard to make a horcrux in the pursuit of immortality, after all, so it's pretty plausible that someone would have come up with a ritual that takes advantage of his soul's already-broken nature, or possibly one which damages his it in ways that matter very little to someone who's literally
torn off a piece already. Voldemort stumbling across an account of such a ritual is slightly harder to justify...but the man quite literally went hunting after such secrets. If one is out there relating to this particular subject and he didn't find it, his reputation as a genius is entirely undeserved.
Thus it's entirely possible that no small part of Voldemort's seeming invincibility during his reign of terror was the fact that even lucky shots that should have killed him and would have killed most other Dark Lords would fail to stick. A killing curse could still take him out, and likely did, with the whole Potter situation, but things like grievous bodily injury may well have just been an inconvenience that required a few human sacrifices to fix.
And as evidenced by Voldemort's failing sanity as the war went on, if he survived losing all his horcruxes and then tried to solve his newfound mortality problem by making
more horcruxes he would very likely have become a mad monster who no one would follow before too long. Which while not
great would still have ultimately united the wizarding world against him and thus eventually solved the problem posed by his existence. It just wouldn't have been as neat a fix as what happened in the books.
