That's exactly how it works! Grid frequency is determined by the speed of all turbines connected to it. They lost a bit of inertia due to energy being taken out

That's exactly how it works! Grid frequency is determined by the speed of all turbines connected to it. They lost a bit of inertia due to energy being taken out of the system (higher demand than supply of power), so they slowed down, the grid frequency lowered and clocks started running behind. To get the frequency back, gotta generate a bit more energy to speed all the turbines back up.