I don’t get how you can take it as serious confirmation that Klingon’s appearance changed in universe in that context. Because it was said by a Star Trek character in a Star Trek episode and words have meaning? I don’t get to pick and choose which lines of dialogue matter and what lines of dialogue don’t, no one does.
Frankly, I don’t get how someone can watch a whole scene and go “well that didn’t actually mean anything for the characters that just experienced it”. It makes more sense to assume that words have their intended meaning, and that Worf’s friends were genuinely shocked to see flat-headed Klingons than it does to pretend Worf, Bashir, and company never actually had that discussion. Like, yeah it’s all fake but main characters are supposed to be real people, the situations are supposed to be real to them…