According to VGC Super Mario Maker sold 3.45 million copies on WiiU and 3.23 million on 3DS (most of the owners of the two versions are probably the same). Given, with digital sales on WiiU might be at 4 million. But selling more double and half in the next installment seems a bit much.
For comparison: VGC has Splatoon at 4.71 million, Nintendo reported Splatoon 2 at 8.27 million copies end of December. So a software that was a new IP on WiiU like Mario Maker, that has sold more on WiiU and got like Mario Maker an expanded sequel. Yet, it failed to reach 10M. And it seems unlikely Splatoon will get there, 2M left is a big chunk of sales to make.
So no, I think it is unlikely Mario Maker 2 will reach 10 million. It is very likely it will sell more than 5 million, it may even double sales, but 10M seems to far out of reach.
Somebody doesn't seem to know how Nintendo legs work. Splatoon will easily cross 10 million. Probably by this year.