Earlier quoted context omitted.
I think orcs eating orcs only happens in the movies. In the books orcs don’t do that. What he missed is orcs eating human flesh
Interesting, although the books indeed don't mention it I don't think it's entirely out of the question. from https://en.wikipedia.org/wiki/Orc_(Middle-earth) > Tolkien indicates that Orcs are "always hungry".[24] Orcs eat all manner of flesh, including men and horses, and there are frequent hints of cannibalism among Orcs. Grishnákh, leader of the Mordor Orcs, accuses Saruman's Uruks of eating Orc-flesh, which they…
As you note, this is good evidence that cannibalism is not practiced by the orcs.
> In Cirith Ungol, Gorbag suggests that Frodo (recently poisoned by Shelob) should "go in the pot"; Shagrat indicates that Gorbag could be "for the pot" for making such a suggestion.
This isn't good evidence; the insult is suggested by the context.
> Shagrat threatens to eat a disobedient orc
Without some sort of context, this is good evidence that cannibalism is practiced.
> and after killing Gorbag he licks his blood from the blade.
But this isn't; blood licking is not unknown among humans either. (To be fair, neither is cannibalism, but they read pretty differently.)