Yeah, it’s literally the same sort of shorthand as “Brit” or “Scot.” “Jap” wasn’t considered pejorative until a wee incident in the fourties. So, it’s offensive because of the outrage that was caused by, well, the Nipponese. (BTW, Eighteenth century Americans used “Tory,” since many of them were also “Brits.”)
That said, “Jap” does tend to be used as a pejorative (although, almost as often not) that is directed to people who weren’t alive in the fourties, so we’ll hold to the standard of avoiding the appearance of offense. Obviously, none of the GWCers would want to give actual offense.
Except towards GR. But he’s in all kinds of denial.