I don't even think it's about job identity, I think Yoshi-P just doesn't like Black Mages in plate armor specifically.

He said the exact same thing in 2016 (near the bottom)

We've had bikini tanks and caster robes for tanks for as long as we've had glamour. Now we've got job icons you can have display next to people's names to show what job they are. Confusion about what job someone is not a valid reason, so it has to be Yoshi-P's personal distaste for it.