Not yet it isn't. Besides, aren't Xbox consoles the #1 portal to Gamepass?
You just answered the main question. Xbox consoles aren't the main platform, but they are an access to the main platform. So for Microsoft to get people on Gamepass, their best access point is a console. That's all the purpose it serves. Plus all those 3rd party and first party sales from people not on Gamepass is just a bonus.
Just a guy who doesn't want to be bored. Also