To me, if my family members are buried near a fruit tree & their bodies are absorbed by the tree only for it to produce lots of fruits after, I will eat the fruit & cultivate the new seeds to grow more fruit trees. When you die, your body may perish but your soul lives on to the afterlife. So to not waste the decomposing body, bury it underground for nature to benefit from it. It is the same thing when you use cattle waste for your vegetable farm. You'll still grow the vegetables & eat them, right? You're not directly eating your family member or poop, as the plants & earth are the natural processor & filter for the non-consumables. It is how we humans survive through nature & it is the natural order of things.
But if cannibalism is the only option to survive, you gotta do what you gotta do to stay alive. But then, it shouldn't be done often or made into a custom, for moral, physiological & psychological reasons. & unless biologically you can withstand the side effects of cannibalism, you simply shouldn't do it. That said, eating another (lesser) species is a means for you to survive so it is permissible, regardless of your moral, cultural or faith standing, as homo sapiens are built to be omnivores.
Ugh why am I writing an essay here? This manga is deep haha. If humans can communicate with (perceived) animals then they won't eat them, because they'll at least be on the same moral standing & intelligence, & therefore won't kill each other for food & will work side by side to cultivate other food sources to survive together. This world seems like it has this kinda setting, besides the high linguistic values it is presenting to us readers plus how big of a difference the humans see the other creatures' cultures & how they cope with it, starting from this one legendary professor & now the student.
Love the soft world setting though. This manga is teaching us empathy to other living creatures via communication & relationship beyond the conventional (human) ways.