I thought it was because Obi-Wan knew Han was boasting about a supposed lie about the Falcon, not that he was trying to lie about what a parsec was.
You would think Luke would also know what a parsec was during that scene as he’s a pilot. That seems like it would be more basic knowledge there than it would be here on Earth.
My gut still tells me Lucas didn’t know or care. The novelization back in the day says “standard time units” instead of parsecs… which would indicate it was an error in the script.
I know Alec Guinness really didn’t like to be known for Star Wars, but because of that… people who didn’t know who he was would look him up and discover his earlier work. That is why you take jobs even if you don’t think the work is worthy of your talent.