If they managed to travel through space, I think that they have found a way to "recycle" organic tissues into a thing that would have been useful.
And appears less barbarian than cannibalism.
More like the "Water World
" point of view.
Cannibalism, as eating directly human flesh (cooked or not), would be at the end in the list of options in case of hunger issues.
After all, nature does "cannibalism" for us (with big air quotes).
Our world is defined, and despite what we throw in space, his mass doesn't vary so much.
Virtually, there's non reason a cell that is part of a beef, butterfly, flower, whatever wasn't composed by raw materials that would have composed another human being.
There is just a larger scale of time, and a bunch of transformations.
« Rien ne se perd, rien ne se crée, tout se transforme »
« Nothing is lost, nothing is created, everything is transformed »Antoine Lavoisier (French too)