Pretty much every major release hits the top 5 of Steam the day it comes out.
The bigger test will be seeing if its still in the top 10 in a week.
And, to repeat myself once more, the intention isn't to assume the PC version is
now selling well but rather demonstrate that there's a high degree of interest in the game even after half a year of availability on a store that is integrated with Microsoft's flagship OS. Clearly, Microsoft was leaving a sizeable amount of money on the table by maintaining the game's WinStore exclusivity, which is something people doubted when Greenberg stated in no uncertain terms that the game would not see a release on Steam, and again when Spencer somewhat stumbled his way through the question of whether or not Microsoft would release games on Steam in the future.
In fewer words, the intended interpretation of this data is not "The game is now selling well", as some have erroneously assumed, but rather, "Microsoft pushing WinStore exclusivity is harming the success of its PC releases". I mean, even the OP says "
hinders success".