My explanation for the book v. film difference on Eridian eating customs is that movie!Rocky was full-on bluffing.
Like he was just calling human eating disgusting, what was he going to do, say "okay actually the way Eridians eat is so incredibly gross we do it in private sound-proofed rooms"? And cede the conversational high-ground and sense of species superiority to Grace??? Nah, it looks beautiful, definitely.
Then Grace says "show me" and Rocky is stuck now, he's committed. So he just bucks up and goes all-in. It's not like he'll see the human after this is all done, he can pretend it's totally normal just this once.
Eventually before they reach Erid Rocky is gonna have to come clean, and Grace will laugh at him for an hour straight.











