Not that I believe the official story, but don’t planes often taxi with power solely from the motor that turns the wheels?
Planes, as far as I know (I worked with 737, 757, 747 and some small prop planes), don't have a motor that only turns the wheels.
That is why they need a tug hooked up to the front wheels to power the pushback. The tug is disconnected and leaves before the engines are started.
The headset operator is connected below the cone to communicate with the captain. (this is what I did)
They tell the captain to start the engine ON THE OTHER SIDE of the aircraft. If they started the engine on the same side they risk getting sucked in.
Then they disconnect and go out a good distance in front of the plain and direct the captain with hand signals to start the second engine, but only if there is no FOD (foreign object disposal otherwise known as debris on the tarmac which could get sucked in) or tugs or planes or people anywhere near the vicinity of the aircraft for reasons shown in the video.
You are the captains eyes and responsible for damage that could occur from the engines sucking in front and on the sides or from the jets blowing things over behind.
After the engines are started the captain has control of the plane and power to move it.
In the video there is no tug and the plane is moving which means the engines would have to be running as far as I know.