Nobody will deny that Touhou is unpopular with the smash crowd, that doesn't mean that a franchise needs that to get in. Joker was never a big rosterfag pick yet he still got in anyways.
Another variable is Touhou being huge among the doujin and otaku crowds, the later being on Nintendo's target for a while (see: Fire Emblem). It would be very profitable if their focus was people outside of the smash circles.
As for why the above would make sense, think about it like this: if you already own a switch and the game, how much extra profit can you make with the DLC? Now imagine aiming at a crowd that does not own either, not only they will buy the DLC, but they will also buy the game and a switch (if they don't already own one), the later also opening room for the purchase of more games to justify owning the system. Ultimately appealing to other crowds (read: not rosterfags) is way more profitable.
Keep in mind that I am not claiming that she is confirmed, my point is that she does have some variables on her favor, and it would make sense to include her.