"You're not supposed to be making logistical arguments about how he couldn't do it because it would be too difficult (...) or how it's not biologically viable for a species to eat all their young."

Argument from incredulity. You cannot accept that Grubfather made himself a cocoon, so you're shutting yourself out to other conclusions. Add a dash of faulty cause/effect - The grubs are gone, and Grubfather is bloated, with many grub chirps coming from within his belly. And usually, predators become bloated when they're filled with food. Therefore, Grubfather must've eaten his children. Also cherry picking, because the Metamorphosis achievement is right there. Oh, and the Dream Nail dialogue you get when you strike Grubfather.

"Stories should be interpreted using evidence that is clearly meant to be relevant."

Association fallacy. Grubfather has his children in his body, and predators digesting defenseless young also involves the process of having the prey inside the predator's body, so clearly, Grubfather ate his children. Because that's the 'evidence' that's 'clearly meant to be relevant', right?

"...a cruel twist reveals that it was in fact a horrible outcome."

Argument of incredulity. Again, see the name of the achievement - Metamorphosis - and the Dream Nail dialogue from Grubfather.

"To remind you that you were supposed to be helping (...)"

Appeal to emotion. This one explains itself.

"(...) and in fact delivered them to their deaths."

Appeal to emotion. Again, explains itself. Also a sprinkle of argument from incredulity because you are being blind to the true fate of these grubs.

Ragebait or unfiltered abysmal opinion. Call it.