I'm not sure if it's been brought up but the best selling Smash Bros game is Super Smash Bros Brawl on the Wii at 13 million. What OP is suggesting is that Ultimate will sell 2 million more than that in a month. That isn't going to happen. Nintendo games have high sales due to them selling for a long time (as opposed to most games which sell the most at launch). Could Ultimate get to 15 million. Possibly as Switch games have done very well and the game has a ton of content. But to say it will happen in a month is ridiculous.
It is entirely ridiculous. The problem here is that it's tbone - in case you're unaware, he's gotten quite a few ridiculous predictions right -, so suddenly everyone's afraid it might actually come true.