No, it’s all ML models. It is very unlikely that an ML model trained on wrist motions for walking and running would falsely report steps for… other activities.
One of the reasons I got rid of my apple watch is because it would congratulate me for successfuly completing my exercise for the day just from turning over on the sofa. It would also tell me how good I slept when I forgot to wear it to bed. So, at least for me, that ML doesn't work
I didn't give it up, but I did laugh one morning at 2:30AM, laying in bed, dozing off and getting startled by the watch telling me I've been sitting for awhile and it suggests I get up and move a bit.
Definitely weird, pretty sure that’s not what most people experience. But such is the statistical nature of ML; your particular movements on that particular sofa fooled it.