It will be available on PC and the Xbox One family of consoles, but I can't think of a good reason for MS to release it on these platforms ahead of Series X- it'll already be diminishing the "exclusivity factor" by being available on three separate platforms, and if they did release on XBOne and PC first, the Series X release would feel like an afternote.
I say they should take whatever extra time they can get if there are any delays on Series X hardware to get the game polished to an impressive degree for all available platforms. There's always room for improvement when it comes to the spit and shine of any video game, and most games aren't released in a sufficiently finished state. Infinite has a chance to really wow the industry with a presentation of stable, glorious completeness. To pull that off on one exclusive platform is impressive in today's industry, but if they can pull it off on three separate simultaneous platform releases? WOW.