Personally I don't think we should assume those are actually perfect magic immunity just because two young wizards didn't know how to react to it. Especially when it's a random McGuffin that pops up and goes away without mention.
Even the better grade immunity of the white king only protected him from wizards blasting his ass, Margaret's death curse wasn't indirect, that was straight up a shaping effect sewing his mouth closed.
If Madrigal of all people can get them then it's strange none of say the Denarians bother to. That whole thing is a weird glitch in the matrix on a lot of levels.
1)Given as the light touch of a Wardensword disrupted it, Id say its not perfect magic immunity.
But young wizards or not, Harry and Carlos were still reasonably deadly combat wizards.
Harry in particular has been repeatedly reiterated as one of the brawniest wizards around.
Madrigal specifically got these after his previous encounter with Harry; it makes sense they are Harry-rated.
2)All magic has its limits.
Note how it didnt defend him against Lara using vampire magic to aggressively re-negotiate their relationship either.
As I understand it, Maggie's curse didnt affect him it just affected his vicinity, preventing him from getting any nourishment before it could get to him.
It would probably be a shaping effect in Exalted terms, but since it wasnt aimed at him, it slipped through the programming.
3) The Denarians, like Molly, have access to innate magical defenses to greater and lesser extent.
Note how Harry has never been able to track a Denarian or Denarian captive with magic. And frankly, their major threats are Knights of the Cross, who dont use magic that can be counterspelled. A Sword gives zero fucks about your antimagic defenses.
I would not be surprised if some of them did put together their own collection though, especially the younger ones.
Nobody is going to look at Nicodemus walking around with a two thousand year old hemp rope necktie of invincibility and not look for their own knicknacks.
But we dont see many Denarians up close in canon, so we dont know how they operate.