As for how they know, as usual the final scene in Star Wars is ambiguously displaced from the timeline of the rest of the film. Who knows when that was.
Agree with everything you said, but ambiguously displaced endings are more of a prequels thing than an OT thing, since in the OT none of the endings require a time jump of more than a few hours.
Well that’s why it’s ambiguous. Just because it doesn’t have to be more than a few hours doesn’t mean it can’t be. In fact, in the case of ESB and ANH, I find just a few hours hard to believe.
Anyway, whether it’s an OT thing or a PT thing or an ROTJSE or a TFA thing it doesn’t matter. It’s not hard to imagine the final scene of TLJ is a bit later.