It's utter nonsense though, because it means that Bashir, Odo and O'Brien have never seen a picture of a Klingon from the TOS era, and never even heard of their appearance change.
Given their numerous and high-profile encounters with Starfleet, that makes no sense at all.
That's the entire point, in my view. As if they were saying: "yes we know there is this difference and we are not going to come up with a neat in-universe explanation to 'explain' away al the problems associated with it, so why don't you simply sit back and enjoy the show?"
I watched back the clip on YouTube and someone remarked that it would have been even funnier had they simply let Worf appear as an old-style Klingon with smooth forehead in the parts of the episode that play in the 23rd century and no other comrade of him even notice the change. Can't disagree with that

But if you really want an "explanation": holophotography must have been incredibly poor in the 23rd century. You can build a case around that with some quotes from this episode and Voyager's flashback. Or, O'brien and Bashir (Odo can be excused of course since he probably wouldn't have had any Starfleet history courses) really couldn't care any less about Starfleet history as they also don't seem to know other basic facts of the time period (uniform color change, mistaking someone else for Kirk), and in general blunder about during their time on the original Enterprise. So it's simply they who were surprised, but Sisko or Dax (for example) would have known.
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