That quote is just referring to the runtime used, which store you bought it from isn’t relevant to that… you can choose to run the Windows store version on the Steam OXR runtime or the steam version on the windows OXR runtime and vice versa.
To the best of my knowledge there is no practical functional difference between whether your launcher is on steam or on Windows store, even though they are apparently technically separate games (eg a purchase on the windows version in game store won’t carry across to the steam version irritatingly).