You don't need that, though, the problem is entirely solvable through the inverters. Having some batteries also helps. Combined it's much better grid intertia than spinning motors.
Watching the enormous progress we’re making on upgrading our energy infrastructure is definitely one of the most exciting things happening in tech. People seem determined to stick to old ideas.
(One of ) the problem with inverter the angular velocity of a spinning rotor is trivially easy to define and measure. The operating frequency of the grid that an inverter needs to target is hard to define under the realtime constraints of a failing grid.
Which is what brought Spain down. It wasnt the panels and windmills, it was the inverters.
And the root cause of the blackout in Spain is still mostly the subject of speculation. AFAIK there has is no official report yet into the details of how things went wrong.