want to turn off set bedtimes

Why then even bother about that they are set?

 Assumed it read things differently when it thought I was in bed, but I guess from your answer it just registers what I'm doing and decides if I'm awake or asleep. Is that right?

Garmin watches that feature an activity tracker will use the sleep and wake times set in your Garmin Connect account to help with calculating some of your sleep information. These times can be edited to improve the accuracy of the sleep information provided.

That isn't to say that you have to go to bed at that time; its just in that time frame if the watch notices your heart rate is dropping and you aren't moving it's going to start calculating sleep. It also controls an auto do not disturb mode, which you can disable if you want to continue to receive notifications during that time.

That said if you are awake and moving around during the bed time, the watch will record steps and other activity still. For example:

The images there are from my own account showing the bed time recorded was almost an hour later than what is listed in my account, as well as showing it was still recording steps, even after the bed time.