Well, cooking terms and botany terms are not the same. Any non reproductive part of a plant is vegetable. But in cooking we have a completely different idea of what vegetables are.
This really doesn’t matter because most people are not botanists and those who are probably know the terms. The only people that care are quirky internet people with debates about weather or not potato salad should be considered a cake or something.
No he’s implying that eating something just because it is “Vegan” is not understanding the point. Vegans usually don’t eat stuff because it’s bad for the environment or because they see animals as equal lifeforms and don’t want to cause them harm. If you don’t eat most animal products because of the environment then you might be ok with eating oysters on occasion. They have a similar co2 footprint as most vegetables. Similarly honey has an even smaller footprint.