Large Inverter Based Resources (IBR) such as huge solar parcs, grid-scale batteries or high-voltage direct-current (HVDC) lines can be programmed to behave like rotating generators, or even to smooth out smaller ripples. They also don't necessarily need a leading grid frequency but can be used to generate their own frequency normal to cold-start or resync a grid.

Only "small stuff" IBRs need a leading frequency from the grid and disconnect outside their safety corridor because those usually aren't controllable from some central grid authority. Thus the stupid-but-safe behaviour mandated for them.