I think it kills the credibility of the designer trying to tell others, authoritatively mind you, that modals are bad while A) I have no idea who this person is or know their credentials B) Why should I listen to them if they're presenting it without a good understanding of design which is what they're trying to preach others?

IMO it totally kills it. Others may not have as strong of an opinion. This is the age old question about separating art from the artist. With authoritative didactics, especially as a stranger on the internet, it is even more important to associate art with the artist so to speak. It comes with responsibility.