Massive improvements to battery technology.

That’s a non answer - what improvement

It is an answer, in that batteries were used for frequency regulation far before they were used for energy arbitrage.

But the real innovation is communication networks and IC control of the inverter. It's completely possible to create a waveform that modulates and responds to variation in frequency in the same way a large spinning mass would. And if reactive power is for some reason not enough, synchronous condensers are very old technology to solve that.

Integrated circuits, basically. The term if "grid forming inverter" and the standards are somewhat new. I'm not an expert, but here is one standard, I don't know if it has been adopted or if others are preferred:

https://www.energy.gov/sites/default/files/2023-09/Specs%20f...

And they have been used where? Everything I can find suggests they are theoretical, not in use even on micro grids yet, and many have huge concerns about them being able to support a grid on their own.

Also the technology required has been around for decades it’s not new and no one’s done it yet.

So again what’s changed because it seems like for now there is no “I have a grid and need something now” solution it’s a “maybe one day”