From what I understand and this is my first Apple Watch (Series4), if you move around during the day, it has to be equivalent to what the watch considers a brisk walk. I walk with a cane and none too quickly at that and my daytime movements do not register as exercise.
I even have a problem with my using the stationary bike at home. Because my resistance is not high, the watch dies not sense my use if the bike, as equivalent to a brisk walk and therefore the stationary bike workout option also does not register. I have to select the “other workout” option, which tracks by duration and does not require equivalency to a brisk walk.