You've got to consider that a lot of people only care about third party games. There is a percentage of people who just wanted a new box that could play the new Batman/Madden/Call of Duty/FIFA/Battlefield etc or just wanted a way to get Fortnite on their TV, and also has the brand recognition from the 360 era.
Clearly not enough to match Sony, but I'm sure there's plenty of people who bought an Xbox that don't even know what the term "1st party" even means.
PU Battlegrounds was also the main title people were playing based on Microsoft press releases in the late period, which was console exclusive back then and that was early battle royale craze. Forums didn't view it as an exclusive but the public sure did.
Madden was also extremely Xbox associated at the time, before Madden lost much of its base, and while Fifa sold more on PS4, Xbox for years had exclusive content. I forgot what it was I think it was a few teams and cosmetics or something. Just enough to keep the console relevant in the late years.
That said I was expecting just reaching into 50, but almost getting 60M is more impressive than I thought.