The switching rate of a good power MOSFET has a bandwidth in megahertz. It's... all solid state electronics. Again, this is just wrong, absent someone showing a cite. It's very weird seeing this kind of seeming ignorance of basic electrical engineering on a hacker site of all places.
Large spinning masses can provide several seconds of inertia. For 2GW of traditional turbine, you would have between 10-20 gigawatt-seconds of energy that is instantly available at any moment to resist RoCoF.