A promotional one-shot doesn't have to be "the manga". This is of course not
always the case, but the burden of proof would be on you to prove the promotional one-shot is canon to the manga to begin with. Otherwise, you'd be presenting a false dichotomy.
There are one-shots that aren't and
are canon to manga's, and if a one-shot is made specifically as promotional material for a movie that can be argued as non-canon, it's even harder to overcome that burden on you.
This isn't to say I'm opposed to the promotional one-shot being canon, after all, I know absolutely
nothing about Naruto. Haven't read a single one of the Manga's. Just thought I'd chime in on this smart-aleck comment.