The reasons why so many people don't believe the "reveal" by Pokemon Smash is because the supposed reveal was just Sylveon's voice actor dressing up and doing some silly stuff to supposedly test type effectiveness. We do not truly know if that is even the ultimate outcome of the final typing reveal or not. She could have been joking around in a little skit that may not have even been meant to be taken seriously.
Having Japanese cultural and language knowledge myself, I'd say that the skit was probably done to stir up debate about Sylveon's type, not to truly deconfirm or confirm anything. I do think that it will be a new type, though.