What? Fish is an animal?? /s

To be fair, the person you're responded to said "vegetarian" not "vegan." But yes, otherwise, it isn't rocket science. My vegan boys are big fans of seitan.

People act like it’s rocket science.

There's always going to be a question as to where you draw the line. For example, is it okay to eat figs, even though they're pollinated by wasps that end up in them? Is it okay to eat plants grown using animal products as fertilizer? Is it okay to eat cultured meat that is many generations removed from a living animal, such that none of the material present now was part of the living animal? How about things in the animal kingdom, but outside the chordates? The ones you'd need a microscope to see? Is honey okay to eat?

There's also the issue that other people that call themselves vegan will disagree with you on what all counts.