MD-80 deploying thrust reversers - GIF

[Mi Tasol]

Whoa! I'm surprised he didn't even wait to touch down, before deploying the reversers. (Almost makes an arrested landing out of it.)

If you read the link you will see it claims that the reversers can be activated at high altitude/speed/etc.

That is, and always has been, pure unadulterated ****.

You will note in the video that the reverser does not actually open until after the aircraft first contacts the runway and bounced back into the air. What is not apparent to the uninitiated is that is a hard landing. Hard enough for the aircraft to be forced back into the air.
At the stage of that first landing the landing gear has not only compressed enough to create enough force in the oleo strut to throw the aircraft back into the air but it is more than enough to close the contacts on the squat switches (properly called the ground/flight switches) on the main gears.
That switch system completes the electrical circuit that operates the mechanism that prevents the reverser handles on the top of the throttles from being lifted "over the hump" and therefore prevents the operation of the reversers in flight. In this particular flight the pilot has pulled the levers over the hump and into reverse in that short time that the weight was on the gear. At many airports crews have one pilot flying the aircraft and the other with his/her hand on the levers ready to act the moment the aircraft contacts the ground as it allows them to exit on a closer taxiway and thus save time taxiing back to a terminal that they passed on landing.

In simple English - on all aircraft, unless both oleos are compressed past a specific point that equates to a significant percentage of the aircraft landing weight, thrust reversers and propellor reverse thrust are locked in the stowed position and the selector mechanism is locked to prevent operation.

If I still had my DC-9/MD-80 series manuals I would post the system diagrams and circuits