I think you're misunderstanding how this flywheel operates. It doesn't provide 50hz AC in the same way the grid operates, it just is attached to a DC motor that provides electricity which can then be converted into AC to power the grid.
That means it can spin at any frequency and it doesn't really matter what state the grid is in.
It's a synchronous rotor which is spun by the mains. When the mains frequency tries to change, the momentum of the flywheel supplies massive leading or lagging current to prevent the change.
It behaves exactly as an un-powered generator idling on the mains.
There is no DC motor.
In the industry they are known as a "Synchronous Condenser" or Syncom. They have been in use for a long time.
Of course it provides electricity to the grid. It doesn't provide any NET energy to the grid, since it's not a generator. It will pull and push energy to the grid in small amounts as needed to regulate the frequency, which is directly related to the voltage of the grid.
That means it can spin at any frequency and it doesn't really matter what state the grid is in.