“or that sunglasses are not OK.”
This is called assuming facts that are not in evidence. And this was discussed earlier.
“It boggles my mind that a Rav or Rebbe would be so involved in deciding that metal frames are OK, but not plastic frames, or vice versa”
This was a case of a Rosh Yeahiva talking about the styles of the time, and yes, that would certainly seem to be within the purview of what he would discuss with his talmidim. (Or chassidim).