Well, most people summoning demons are too self-centered, egotistical and narcissistic to actually ask. So lying rarely comes up. Everyone is pretty dead set on their preconceptions of Good and Evil, so they don’t think to ask.
Obviously, if a demon were to bring it up, anyone hearing it would most likely dismiss it as a trick or lie or obfuscation.
As to what other immortal types think; I have no idea. I presume that most archons/angels/avatars assume demons are sort of like themselves and got there that way. What the gods themselves know? Well, I have no idea. Some of them, obviously know what’s going on. Plus they all got to be gods someway or the other, so one would presume they have at least a basic handle on the whole “mortal vs immortal” business. Particularly if they can ascend mortals to become saints and archons.
A lot of what happens when eating depends on where you are being eaten. Keep in mind that in the Abyss, disassociating matter into raw elemental forms and harvesting the mana and animus takes a lot of energy, for those that can do it.
Eating another demon or immortal type is easier to digest and takes less mana, so you get a better meal of mana and animus. But it’s also much more dangerous. If you try to ‘digest’ the animus/lifeforce/soul of someone of near equal or greater power, you could end up getting eaten yourself…
But for example, if Lillith decides to eat someone, she’s probably only going to try and eat someone that won’t give her indigestion and that she can overpower, steal their mana and absorb their animus, i.e. fully consume the person’s soul. And that can be tricky, sometimes some of the animus or soul can escape. Sometimes, there is enough to infect the eater and well things get rather weird.
I’m aware of a couple cases where you end up with a hybrid being, often with multiple personalities. I know of one being (or actually a couple beings) that in fact go around consuming others partially to create sort of a hive mind creature.
The matter consumed is generally re purposed by the consumer into their corporal form; this is a cheaper way of acquiring matter than pulling it from the material planes.
Not all demons can digest other living creatures though; and some only digest parts. E.g. there are soul sucking demons and mana sucking demons that leave the other parts alone and don’t eat it.
And finally, eating someone on the material planes is very sloppy/messy and many times the soul and some mana escape complete absorption.