You can (and should) also turn it off:
support.google.com/a/thread/299...
Google could have chosen to label those replies with “composed with AI.” That they didn’t (and that most people would feel the ick if they did) gives the game away.
I find Gmail’s new “write this for me” feature deeply sinister. They’re trying to convince you that you are dumb and helpless. Don’t let them steal your ability to formulate thoughts and communicate. You were capable of writing an email in 2022 and you’re capable now.