This seems to be fixed now...App Store is no longer saying my MBP is "not compatible" and I can download Xcode from the App Store again.

Yup...there's an earlier thread below of people reporting this issue.

Same problem. MBP M2 Max on Sonoma 14.2 and Xcode 15.1. Thinks my Mac is not a compatible device, sigh.... My workaround: I downloaded the Xcode .xip from developer.apple.com and installed manually...seemingly fine. But the AppStore install/upgrade path is definitely broken.