The Ork upgrade Lotsa Thrusters does not apply to Mega Roks. These are battleship class units, and other upgrades that apply to battleships DO apply to them. Either this ability is not being applied correctly to Mega Roks or the text is incorrect. Video showing the issue:

Youtube Video

The Mega Roks have the Lotsa red button so it would not benefit from the Lotsa Thrusters upgrade (reduce the cooldown of Big red button) so it's normal that it doesn't have it.