I agree that it’s about as good a retcon as one could hope for; given the corner they’d been painted into; it just seems like a deeply awkward one in context:
If you are trying to impress some yokel whose hyperspatial navigation skills you openly disdain; a boast about speed that relies on the other party being familiar with the hazardous interstellar phenomena of a particular smuggling route; and being familiar with how a ship’s hyperspace capabilities constrain its performance in nontrivial gravity wells seems like a very, very, odd choice.
As a solution to the ‘measure of distance, not time!’ problem, I certainly can’t think of a better solution; but as a ‘social pragmatics of Han Solo’ problem, it falls a bit flat. Despite the boast only making sense in the context of some fairly specific knowledge of interstellar navigation and criminal activity; Han delivers it completely straight; and Luke expresses no indication either of puzzlement or of understanding.
That said, this sort of conveniently-chosen-level-of-common-knowledge seems to crop up from time to time in Star Wars: the galaxy is simultaneously large enough that you can always find an entire solar system to hide something in(Echo Base, Second Death Star construction site); but nobody ever betrays the slightest confusion or unfamiliarity about locations, despite the galaxy being a huge place, apparently with poor documentation (“My lord, there are so many uncharted settlements; it could be smugglers…it”), where place names are terse and nondescript. Grand Moff Tarkin knows immediately(despite the site presumably having been out of the way enough for the rebel base located there to be used and abandoned without contact with the empire) that ‘Dantoine’ is plausible but out in the sticks somewhere; “You must go to the Dagobah System” apparently narrows a search down to a few hectares at most, on a single planet.