This is correct, doing a workout with ERG will hold your power very steady. You can see if someone is in a workout (may be using ERG or not) if there's a bar graph on the left side of their name banner.

As for the higher than average w/kg for a longer than average ride, it doesn't take anything but a few keystrokes to shed weight in Zwift. It's best to just ignore what other people are doing on there most of the time.