Hmm can't say I entirely disagree with them on that one. I mean it's clearly not a harmful phrase but it definitely is a useless one.

It carries almost zero information. Who is going to read "trigger warning" and think "oo they know that I'm highly sensitive about this specific unknown subject. I don't want to get triggered, I'll stop!"

Contrast it with something like "spoilers" where everyone agrees on what it means and people generally really don't want to read spoilers.