Slop.

Because that's what they do, they mimic emotions. They don't turn it off when in private because they aren't scheming people, they are monsters that mindlessly mimic human behaviors. They act like people when they're in private and when surrounded by others of their kind because all they know how to do is act like people. The same way a mimic acts like a treasure chest when there are no people around. It's not a person who decided to act like a treasure chest, it evolved to act like a treasure chest because that's what allowed it to eat people.

There is no reason for a demon to think to himself "I despise geniuses", because there is no circumstance where it should be able to fool anyone, and before you say "telelpathy," if you're already doing telepathy on them, you would probably be able to see the lack of emotion, as evidenced by the Macht backstory where the magic item didn't detect any malice from him. Speaking of Macht, he clearly displays emotions, albeit evidently not malice or guilt, because his, uh, termination of contract is clearly highly oriented by affect and not some plan. He clearly has a meaningful inner world, or he would have pursued a solution that didn't put a huge target on his back and ultimately trap him in the city.

And again, the demons are extremely intelligent creatures, around the same level as humans, and exhibit vastly different behaviors, this isn't merely an insect being strung around by a series of reflexes. They literally are capable of reflection and make choices based on desires that are counter to what they know is their better survival strategy.

But you can't believe that it would be possible for a creature to exist which similarly lacks those mental faculties but appears to have them?

I think that you need to establish a motive, since these are intelligent beings who are shown to deceive with intention, lying according to a prepared story, even if they have some natural proclivity to just sort of ingratiate themselves unconsciously anyway (Macht's backstory), because they display emotions in circumstances where it's just a waste of energy to perform or where there are clearly much greater concerns for their ruse than how they feel (like the specific nature of their conspiracies).

The only conclusion that you can draw here is that it's just word of god because the story does not support this interpretation aside from when it says "we used magic to look into your brain and here's what it said". P-Zombie predators are a garbage concept, is what I'm saying, because if you are keeping up a ruse even when it's actually counter to your survival and you know it, it's a shitty survival strategy, and inconsistent with the fact that they do also sometimes drop the ruse when they find it expedient. But there's no line to demarcate between making one choice or the other, so it's just whenever the author says so and we're instructed to just pretend it makes sense because the magic bracelet (i.e. the author again) also said so.

Edit: Also I just hate p-zombies anyway for basically the same reasons, because they are question-begging not having internality when clearly there is a causal basis for being totally consistent with being human. This is most egregious of all with a line like:

They are literally no more internally complex than slimes, they're just good at convincing you otherwise.

Because there is nothing you can tell me to say a p-zombie or a demon is incapable of complex thought based on an array of memories, inferences, and other factors, because whatever process takes the input and turns it into the output that is observationally identical to thought must itself also be a type of thought, even if it's using punch cards rather than circuits or circuits rather than neurons or "magic" rather than any of it. It is still processing information with just the same level of complexity and diversity as a human (perhaps one with an on-and-off emotional disorder in the case of demons), and indeed exactly the same type of information (same caveat for demons again) and exactly the same output. It's like solving a math equation with the same answer in two different ways (neither of which is random) but saying that only one is actually "doing math."

This is loosely the basis of the Turing Test, and obviously we've developed since then, but it's still just the same "ghost in the machine" mistake as Turing was addressing then.