I think Vanguard is going out of their way to make it confusing as possible. For most games with a Series X and Xbox One version, the disc contains both versions and will install the appropriate one when placed in a system. Vanguard, however, doesn't work that way. Instead, with that one, if you buy the one marked for Xbox One and put it in a Series X, you'll be playing the Xbox One version on a Series X, not the Series X version; The box is just marked Series X because technically the game is playable on the X via backwards compatibility. But if someone buys the Series X version, it then has the "bonus" of the Xbox One version on it as well.
So, Vanguard should have 2 entries in the Xbox One section, one for the Xbox One version, and the other for the bundle of the Xbox One & Series X versions.