If it doesn’t make sense for people to say “vegan except for chicken”, “vegan except for beef”, then why on earth would it make sense to say “vegan except fish”?

I don't know why you take offense to someone saying "vegan except for beef" or "vegan except for fish." There is nothing inherently wrong with these statements. It just depends on whether you view veganism as a lifestyle or as a diet, which can vary depending on the context. In the context of a nutrition study, we are obviously talking about the diet, not the lifestyle.