We did that last year, and NO you cannot set foot on Cuban soil...tried that! The Varadero stop is only on the return leg of the trip and usually it's not a long stop, but last year we left MoBay early and when we landed in Varadero, the crew was scheduled to change and the new crew hadn't arrived yet!!! Our short stop turned into about an hour, so the time we saved leaving MoBay early was lost in Varadero. Apparently, the new crew strolled into the Varadero airport, on time, and heard their flight being called over the PA system, and took off running to get to the plane! They were quite frazzled when they stepped on board! lol
We did the flight but went to Cuba so had to wait at MBJ...that was a challenge as we were there and couldn't get off the plane. You aren't allowed to get off the plane at all...they let you walk the aisle but it is hardly the same.