Yep, it is pretty unbelievable. I don't think anyone would have guessed this game would get this high ever. And it's not even done yet, it has some more million in it, and yeah even before a potential rerelease.
I remember hoping that one day a game in the series, any game, would reach 10M "because the series deserved it", and then hoping A Link Between Worlds would finally be it maybe due to the retro/nostalgia effect like what happened with NSMB. Seems like ancient, and laughable, history now. I'm glad with with how it worked out though, Breath of the Wild is the perfect game to have widened the fanbase as much as it did and take the franchise's crown. I think it's unrealistic to expect Tears of the Kingdom to match BotW's sales, excellent as it may be, also keeping the late release in Switch's life in mind, but I hope the fanbase has indeed widened enough for it to at least get to half of these sales.
I think 15M is a given for Tears of the Kingdom, can't see it doing less than 20 million honestly. But it will fall short of Breath of the Wild's numbers.