Very odd. TMo here too and I got the same update, but my Security Patch is dated October 1. Locked S21 5G. I was a bit surprised because it almost seemed too fast for TMo. Been running late by at least a month for some time.
Roadijeff has a factory unlocked variant of the S21U, which is different firmware, whereas you don't. There are some differences in when factory unlocked and carrier locked devices get updates. Generally, carrier locked devices always get them first.