This is true, but also from what I've heard the save file times don't count the time when the game is paused. I'm not sure if they don't count pausing during animations/cutscenes/"do you want to X" questions, or if it just doesn't count time spent in the pause menu, and I'm also not sure how that applies to multiplayer. But the point is that the save file times may not be accurate either.
IMO, if you're not prone to leaving the game open while not playing, the Steam time is probably more accurate. Neither one is great though.
Also, my Steam time is just over 3k hours, but I know I left the game open on the title screen for a significant chunk of the first 1000 hours, but also that doesn't count my mobile time, so I'm estimating my play time is around 2500 hours. (I can't really add up the save file times because I accidentally deleted my mobile saves a while back and I have some save files that I moved out of the save folder because I'm not playing on them anymore.)