In terms of destination weddings 6mo is last last call, is is possible that they have you a late invite so you'd have an "out" and they're actually hoping for more of an elopement type wedding?

generally giving out an invite where the logistics don't make sense is a courtesy invite (I don't agree with it, but it's a thing).

the fact that you're asked to actually /be/ in the wedding raises flags, unless you're misconstrueing the invite, being asked to be a groomsman definitely involves a year in advance because you usually have to get a suit to match and then tailor it.

you're fine to just say no, because the couple obviously didn't get the timeline in order