This is written like the author has very carefully read all the Wikipedia articles on the electric system. But they mentioned the importance of inertia, so I'll give them +1 for that.

At the moment, many of the regional grids run on different frequencies, meaning that electricity has to be converted to direct current and then back to AC before it can be shared between grids. This limits the ability of neighboring states to benefit from any excess generation in California and limits California's ability to manage the power on its own grid.

"Many?" I'm not sure three is considered "many," and I seriously doubt the eastern border of the Western Interconnect at Texas/Nebraska/North Dakota is limiting the ability of California to manage its own grid. The famously poor ties between Southern California and Arizona will limit things long before the asynchronous tie another state further east.

if we want to see how it will end up playing out, we can just continue to watch California.

I'd watch Hawaii. A 100% renewable mandate on an island is aggressive, and the solutions they come up with will be able to steer everyone else.