I sometimes use a class 1 electric mountain bike (250w, assist only while pedaling) and that is a fantastic workout...provided you are riding 10-15% grades uphill and (for the upper body) going down something similarly steep.

A throttle powered class 3 ebike on a flat paved path is going to be a different story. Likely less of a workout than a walk. Much better than a car though, for both the body and the planet.

Of course, assuming terrain allows options, one could alter their route to get a better workout. Reducing the assist would be an option too.