What if Denji eats the chains devil and then has one hell of a time fighting until he's forced to throw up a bunch of stuff?
As for his true identity, I'm gonna guess it's not just being forgotten but specifically Damnatio Memoriae: Pochita was in such a weak state beacuse the idea of being erased from history as a punishment is not only kinda historic but also greatly weakend by first the invention of the printing press and then the internet. But of course, it's still the most terrifying thing for every other devil because they run on fear. It's not that they are forgotten because Denji has eaten them, it's that Denji causes them to be forgotten and they stop existing as a consequence, until reinvented. This is a different kind of death for them than being in the cycle of reincarnation.
I think it kinda works with Denji and Pochita's relationship. Pochita has someone who will remember him forever, and Pochita wants him to be normal rather than becoming a significant enough figure as to ever risk Damnatio Memoriae himself.
Anyway. That's my crank theory. I look forward to being wrong and the 0.01% chance of being bang on and this comment being rediscovered in 10 years and being hailed as a prophet. I fear, however, that my presence here will one day be forgotten by everyone...