This seems extremely hubristic to assume that Matty can't be fooled by illusions ever.
Especially since we literally got fooled by the illusions over the Slaaneshi Slayers at Vlag until a miscast-bomb tore it away.
This whole situation is very surreal, with Mathilde's narration going into unusual levels of misleading/outright-false poetic description (
"Karag Dum's a giant Herdstone! I mean, I can't actually see Karag Dum, or a Herdstone, or literally any magic at all, but Karag Dum is definitely a Herdstone!"), plus already witnessed illusions such as 'heat haze' multiplication of Borek as he left.
The fact that there's literally no description of the Winds at all, not even it being 'too confusing to understand' or the like.
Morghul acting completely unlike Morghul, from his level of intelligence, to his restraint, to his holding back of the eternal uncontrollable automatic corruption aura.
The dwarven Waystone network being uncorrupted.
Dwarves being willing to disown people for much less than active Chaos worship, Borek may think their sins unforgiveable by Dwarves - especially the hyper-traditional dwarves he was trying to get help from for centuries - without Dum being considered unworthy of rescue or aid by us.
Basically, we've seen just recently the kind of fuckery that can be done with the power of a Karag Waystone behind it.
We've fallen for illusions generated with
much less power.
We are literally seeing visual-illusions we know to be false already.
The things we are seeing don't add up.
Our normal level of Windsight is going completely undescribed.
We haven't even tried taking a deeper look like we did when first investigating Vlag, let alone a spell or dispell/miscast-bomb like the one that destroyed the Slayer illusion.
Mathilde's thoughts that things don't quite add up seems like an IC nudge from Boney, especially given it comes after 99% of the thread wanted to just bail immediately.
Not saying that the true situation is good, or even that it's better than what we're seeing here, but it's clear that we're missing things.