Doesn't really matter if the cue itself is canon, because Obi-Wan's reaction is. I only brought up the cue to show that, even in the 70s when the movie was being written, the intent of the scene was that Han was speaking nonsense.

It is kinda weird that they use Parsec specifically, but Parsecs are a thing that exist in Star Wars (Padme uses the same word in AotC, referring to a distance). That's also a different problem from the one you originally outlined (that Han uses a measurement of distance instead of time).

But I basically just see it as translation convention. The characters use an Earth-based word because the film has them speaking in an Earth-based language, even though the actual in-universe language of Star Wars is Galactic Basic, not English.