Well, to be clear, he is a truth specialist, how keep in mind Lehm's power is assumed to be one power level above normal (although it could be more). This means his abilities may or may not reflect actual reality. That is, they may reflect reality only to the limits of his emotions not being in the way, not necessarily recognizing the limits of himself or of reality in general. Or to put it more directly, his observations may be acceptable for basic things like finding a match to his experiments, but may have bias or outright delusion when it comes to reality itself.
For the sake of argument, let's assume he's partially right. From what we seem to have observed so far, there are microscopic organisms which are capable of altering matter mostly by chemical means (and consuming live organisms, based on that Decay spell). It also seems like that demons of the Vorpal Sea thing appears to be more correct than we thought. It's even possible some of these have supernatural abilities, and by extension it seems like they have altered humans in some way. It's possible either of these ideas have some merit, although it seems unlikely that both do. We have heard that there is no evidence that the brain is altered, so it possible that it is (a) an increase in latent magical ability that everyone has, that is the dissolving test actually is just a placebo, making people aware of more and thus able to do more, or (b) it installs some sort of remote control implant which allows the Qualia to receive a stronger signal.
The problem with b is that things like magical efficiency don't really make much sense, nor do much of this scene (why would these critters bother to show up in the bottom of a pit)?
The other option is that it uses a sort of mental energy (similar to PSI or faith), which is internal to the caster. To explain it in simple terms, it's like Flat Earthers. It's possible to awaken to a mind-blowing idea that changes your whole paradigm (dissolves what you believed before). Before, you were like "nah, man, the Earth is round." Suddenly, after you believe the new thing, you may find events reinforce the belief ("I can't see past the horizon", "that plane had to go north to get from US to China", "my friend seems to have dropped off the face of the Earth, maybe it's not my crazy behavior and they actually did"). True or not, paradigm changes create a sort of personal bubble.
Both theories account for the insanity exhibited by high level types that haven't mastered the later dissolving tests. In the one case, the paradigms you believe have no sense of self or reality, meaning you are literally reinforcing crazy ideas (very similar to what seems to be happening to Suspiria) without having a sense of "let's verify this." In the other case, without the proper control device, the mental energy of the person has developed, but the Qualia are not properly in control, and attacking the user's brain (as in actually chewing on her psyche).
Interestingly, qualia literally refers to stuff that is observed
as different for everyone. That is, I have no idea whether you are reading the same thing as I'm trying to say. This could well explain why Thin Man believes in Qualia, whereas Melter is talking about her perception of reality. Maybe it
is that subjective.