If someone adopts a vegan diet purely for health reasons, are they not a true vegan? They haven’t accepted the moral framework that killing animals is wrong.

Should the study filter out people who eat entirely plant-based for reasons other than ethics. because according to you, that’s not a true vegan diet.

I think you are largely correct that veganism is a lifestyle, not a diet. But in the context in which my comment was made, it was perfectly reasonable for me to say “vegan except for fish,” because the study was analyzing veganism as a diet, not as an ethical lifestyle.