Definitely different pieces, but the anecdote is still accurate, sort of.

As I heard it, RVW had to make so many revisions to the harmonica piece (it was a new and untried instrument to him) that he jokingly threatened that if he had to rewrite it one more time, he'd rescore it for bass tuba.

Whether he already had one in mind, or whether this remark inspired him to do so, I don't know, but he did produce a tuba concerto a couple years later.