There are also custom scheduling policies in the XBox One IIUC. I agree that a “custom version of Windows 2000” for the original XBox is wrong, but it is not extremely consequential in this article, I can see where it comes from, and I don't think its erroneous enough for it to be a big deal to be presented like that to the public; only experts will find the distinction interesting.

Saying it is a custom version of Mac OS, I would call it absolutely wrong. Here, just wrong is enough :p