There is also another aspect of it - Mathilde seen stream of magic flowing in and 'ending', and stated that it had to be used. But if is was being gathered and shaped into ritual, then this enormous amount of energy gathered over centuries would be much easier to spot than merely incoming stream of magic. A hell waiting to be unleashed. She sees nothing... but is still on verge of panic, convinced of a danger.
I mean, if they were just dumping it into the magical equivalent of a big open-top pool of glowing radioactive goop, probably it would be super visible, sure. But the reason that Mathilde cites in the text for concluding this is a ritual is that she thinks the only way there wouldn't be the magical equivalent of a hyper-irradiated nuclear meltdown site that would be clearly visible even though stone is an excellent insulator of magic is if the magic is
not just pooling there/radiating out randomly.
I.e., her conclusion is that this torrent of magic has been
actively channeled into something for this entire time. To use an analogy, she thinks the reason that there isn't a big pool of toxic magic down there is that something has been burning it for fuel as quickly as it pours in ever since Karak Vlag got stoneified. And even if it is being stockpiled, there's no reason to think that they wouldn't be storing it in something that's got the magical equivalent of lead lining (whether that be in the form of special materials or specialized wards) to tamp down the radiation leakage drastically, and hence limiting its visibility to Windsight commensurately.
Also, y'know, it's hard to see what anybody would get out of magically convincing somebody who sees through the first layer of obfuscation (stone is an excellent insulator of magic) that there's actually a dire threat when actually nothing important is happening down there. Like... what's the motive supposed to be there, exactly? Do you think Ranald is gonna pop out, yell "psych! lol gotcha noob" and then scarper off back to the divine realm snickering to himself because it was all one enormous practical joke?
Infohazards are spooky by nature, but I don't think we need to go out of our way to invent reasons why this one is spookier than it already seems.
The best-case scenario is that whatever hostile is here thinks it's better off letting us go than ambushing us, and you're staking a whole lot on the belief that it's wrong.
The first implicit assumption underlying this is that they've already noticed us. Which has definitely not been established. The second implicit assumption there is that the only way they wouldn't attack immediately is if they've decided to not attack us at all. Which has also definitely not been established.
They could (easily) have failed to notice us, as we've done very, very little to draw attention; we literally haven't even cast a spell up here, we just used our Windsight which is the magical equivalent of passive sensors. Channeling
Ulgu to cast a spell and using that spell to summon a dragon and a bunch of knights, on the other hand, is attention-getting as all sweet fuck.
Likewise, if they're prepping an ambush, it rather likely is not meant to be for this exact moment. Which would mean if we don't show our hand, we can get back to the Expedition and give them forewarning
without letting on that we know it's coming, which is a significantly stronger position to be in.
