I think I might have an idea where the gen 7 starters got leaked from.

In a recent interview with Masahiro Sakurai, he revealed that the project plan for SSBU was made in December 2015. It's usually around this time where the Smash roster is planned, if not finalized. It's fair to assume that just like last time with Smash 4 where he had a spot open for a gen 6 pokemon, he left a spot open for a gen 7 pokemon. May to June 2016 could be a good time for him to ask Gamefreak to send him reference sheets so he could pick a gen 7 pokemon from these.

The date when the starters got leaked? 6/4/2016.

My theory is that the starters was leaked by an employee in Sakurai's Smash development team. The timing is just right enough for that to happen. The detail that makes me believe that is that Incineroar is shown performing some kind of an asian mist move in its reference sheet, which was seemingly scrapped (both from the game and from the concept art made available to the public). I don't think that they would have sent the sheet to the anime team if it included a scrapped move.

I can't explain why they would scrap the move after showing the concept art to Sakurai though.