If you look at my sibling post I explain that is how they used to do control, but they outgrew it. The spinnup times made it act like a spring damper system and it had bad control states even then.

Yes, there's always been people phoning power plants and asking them to make bigger changes than the small automatic self balancing changes, but at the micro level the grid drags thermal power plants along with it via governers.

Most control still works this way. See the postmortem on the Iberia black start. Renewables that were trying to do frequency following drifted out of sync and injected misaligned power into the grid. They couldn't even figure out where it was coming from their telemetry is so bad. Then they were asking the gas plants to spin up to add inertia but it was too late as the boilers were cold. IT and other smart grid ideas were conspicuously absent.