This discussion has taken a weird turn. It's not fake information, neither is it secret, inside information.
Every month Map Camera releases this information, presumably as promotion, and it is picked up DC Watch. Here is the article on the February ranking. If you scroll down, there is a second list which is used cameras sold. I guess this is where maw found this information.
Map Camera?2???????????????? - ???? Watch
As you might imagine from the cameras in the list, Map Camera is one of the more serious camera stores. People without an interest in photography would not know of it. So it is an interesting measure of what enthusiasts and professionals are buying for the month. Incidentally, if you find yourself in Tokyo, it is a great store to look for used or new camera gear. They have loads of stuff and are used to dealing with foreigners.
What is clear is that D850 is doing very well for Nikon. I think it makes sense because it has the kind of improvements over it's predecessors that would justify an upgrade. It's versatile enough that it could serve as a logical upgrade from several of Nikon's previous cameras.
If you want to get a completely different view of Japan camera sales, the BCN Ranking for the last month is completely different. This covers much more of the market, but more general electronics stores (like Best Buy, Target in the US). The items in the list are very different. Impressively the D850 is at 21 even on this ranking. However, Sony A7/A9 are nowhere to be seen.
????? ????????? - BCN RETAIL
There is no single ranking in Japan that can tell categorically which camera is selling best, but if you look at a few, you can get an idea.