It looks like T-Mobile isn't actually preventing or blocking Private Relay, but that it doesn't function while T-Mo's Content Filtering is enabled. Here's more information on that.
Good point. I upvoted your comment, and I wish I could edit my comment to add this. It seems that I can't edit the comment anymore though.
PS: The 9to5mac article has an update which indicates that the blocking is _not_ exclusive to users who have "filtering and blocking features enabled".
https://tmo.report/2022/01/t-mobile-blocking-icloud-private-...
tl;dr - disable T-Mo Content Filtering on your account and PR will work just fine