Oookay. Lemme explain why I asked:
https://boards.4channel.org/v//thread/474556395/new-information-for-smash-ultimate
As you can see, this post was made about an hour after... er, the "Impact leaker who we totally have never seen before".
Thing is... that account barely had anyone following it before Sabi's retweets. So it would be weird if this was just copypasted from an account that (as far as we knew at the time) is just a random person.
I normally take these things with a huge grain of salt (and I still am taking this one like that), but it certainly is weird. The exact release date being there caught my eye.
Luckily, this rumor has an expiration date (September 24th), so we just need to wait a month to see if it sinks or swims.