It's apparently coming out shortly (like, this month or next). But, more to the point, the delay apparently wasn't because a platform vendor purchased it to be an exclusive, but because the dev team hit some kind of technical problems with the port. That is, it's not in the group of "Mario and Sonic" exclusives used to sell a platform, and Microsoft's acquisition was to make Starfield one of these.

https://www.techradar.com/gaming/xbox-series-x-s/baldurs-gate-3-will-release-on-xbox-between-september-and-november-according-to-swen-vincke

EDIT: Split-screen on the XBox Series S is apparently where the problem is:

Larian has been struggling to get Baldur's Gate 3's split-screen co-op feature running smoothly on the Xbox Series S. Despite the feature working as intended on Xbox Series X, Microsoft policy demands that Xbox Series X versions of their games cannot have any features that Xbox Series S editions lack. This means that canning the feature on Series S simply isn't an option for Larian.