In recent years we even get statistics like our playtimes, down to the percentage of docked and handheld mode usage, made available to us at the end of the year, so it wouldn't be surprising if they'd know the amount of active players.Uh, Nintendo and TPC definitely have very accurate data for the number of switches/profiles that have played any given game on a switch with any sort of internet connection, regardless of online functionality in games.
All the first party companies will be collecting that kind of data.
By the way, I wish they would send those statistics out in January, since they don't seem to count December, because otherwise SMTV would've likely been in third place for me.