Well, obviously that was an advice given by a working man trying to punch above his weight and be treated seriously in situations where most people from his background weren't. It worked for him, and it worked for me when I was looking for jobs, houses, had to do official business and run a company. I suppose a situation like you described has never crossed my dad's mind because it's just such a unique problem to have. As with any advice - use common sense?

Ideologies defy common sense; if someone believes a suit is always appropriate, they might not accept that there are contexts where it isn't. Likewise the other way around.

I remember when a candidate came in wearing a suit, and our VP Eng said, "suit eh? Guess he didn't do his homework."

Seems very petty

For some roles, it's not petty. For example, for a sales role, you'd expect your salesman to be able to fit in at a client right from the first visit. If they can't do that at your company, that's a negative indicator for them.

Social signaling works both ways, and yes, in both ways, its petty.

Aim for the middle.

Black pants, white shirts. Works everywhere.