Cadence sensors will have the avatar in-game emulate the speed of the actual rider's legs - if they coast then the avatar stops spinning. Zwift tends to "brake" a little when no power is applied - this helps people who want to stop riding slow down (since there's no actual braking in-game). In early days there was no braking so you would just coast forever downhill for a bit, especially as road resistance was pretty minimal. Now, not so much, hence the pedaling.
Zwift racing is quite a bit different from outdoor racing so it's fun to see the pros adjust (who haven't been on the platform).
Full disclosure: I work at Zwift.