It was the original RC build that was installed on the new iPhone 13 models. The new update was simply a minor update to iOS 15. If it didn’t come with iOS 15 installed, the download size would have been 5+ GB in size.
The answer to your question is: no, 15.1 is not out. It’s out in bets form only and will not be released publicly for several weeks.
What you are seeing is an update to 15.0.
As the previous poster alluded, some iPhones were apparently shipped with an earlier build of iOS 15 than what was released early last wee.
If you were following along, the Release Candidate software that was released to developers immediately after the iPhone event was not the same build of software that was actually released to the public last Monday).
Your update was to move you from the Release Candidate to the actual released software.