It can be either or both. Whether other animals or people cause suffering to animals isn't a statement about whether it is ethical for people to do so (naturalistic fallacy).

In terms of strict definitions of what should or should not be eaten based on its suffering, I think that's much harder to do. There's always going to be some gray area. Plants respond to stimuli and try to protect themselves. Jellyfish and insects and cultures cells are on a spectrum where it may not be clear how to draw the line.