Steam's numbers are more reliable than what Asobo recently offered in his video for a reason. A large percentage of players according to Asobo were people playing in career mode so it's not hard to imagine that many were casual players playing with the game pass. Those same players will disappear sooner rather than later as with any game these days. 

The people on Steam are people who are interested enough in MFS to have paid full price, of course not everyone is there but it can be a good indication of the health of a game.

There is also a missconception about steam-statistics. They are not reliable to estimate the total number of users of a given sim (using other platforms) but withing its community I don‘t see any reason why they wouldn‘t be reliable. It’s the same source.