They aren't going to let one person buy a disc, then share that disc among all their friends and they all get a copy.

Like I said, I'm reading between the lines a bit, but here's another source from before the official announcement:
https://www.purexbox.com/news/2026/07/report-xbox-disc-to-digital-feature-will-allow-you-to-digitise-your-physical-collection

The only potential issue is with certain Xbox One discs - Microsoft has apparently been telling testers of the disc-to-digital program that "it all depends on how and when the disc was manufactured"

the discs will still work even after you've digitised them, and the digital entitlement won't disappear unless you loan the disc to a friend or sell it on permanently.

Maybe "serial number" isn't exactly the right word for what's happening, but they apparently have some way of fingerprinting the disc itself so they know when it is being used by someone else.