I have to correct myself, it was the Nintendo 64 that finished at ~5.5m. I thought the N64 was at 5m and the Saturn at 5.5m, but it's ~5.5m and ~5.8m, respectively. Either way, Switch has long passed the Saturn. I don't think there's a reason to doubt the Saturn number because Japanese hardware tracking has existed during the time Sega's console was on the market.
Oh well then we can have a thread when switch will pass PS3 in Japan :).