Kaymack- you correctly interpreted those videos. Eddies in the wake are caused by low pressure existing there...the eddies are air filling in that vacuum. An abrupt end (like a boxy trunk on a car) is a very good way to create a lot of drag. Smooth entries and smoother exits are what is necessary for low drag. Take a look at a racing sailboat hullform- smooth in, smooth out. The waterborne portion of the stern is NEVER squared off.

Our spoiler delays the onset of turbulent flow to be further away from the vehicle, nudging that airstream 'up and over' the sudden change, exactly as you guessed. This lessens the abruptness of the transition into the wake/low pressure behind the vehicle- spreads it out over a larger volume.

Think about trim tabs on a boat, and flip it around in your head- you can leave a really big hole in the water behind you to create a rooster tail and make it good for surfing, or you can make less of a hole, flatten the boat, ride smoother, and save fuel.

There were a few Navy ships fitted with fixed 'tabs' on the stern at the waterline, similarly to our spoiler. They actually angled down a few degrees (flip it in your head for our car picture), and did improve fuel consumption a few percent. It's real.