I'd always taken it to be Kirk telling a joke. I mean, just think of the odds of the glasses in 1986 wending their way through two hundred years to end up in the antique shop where McCoy could get them. (Heck, what are the odds of their even surviving two hundred years? Kirk managed to break 23rd-century prescription lenses on them in under a week.) This isn't a Somewhere In Time scenario where the time-looped object has a compelling provenance.
No, it's not. It's Star Trek, where people in entirely separate universes manage to know each other and have jobs or roles similar to those of their counterparts. Given how the entire series mucks with the laws of physics and probability, it's absolutely plausible that the glasses could have been part of a loop.
--Sran