Reading about this for the first time, it sounds like this problem would also apply to the Synchronous grid of Continental Europe [1] discussed in the press release. This is approx 4000km across, and much more if you include Russia (I'm not clear on the extent of the grid). It does not sound like these are split into individual grids.

Maybe active synchronisation solves this, but to my (inexpert) mind the network would still suffer from interferences at various points between power plants.

[1] https://en.wikipedia.org/wiki/Synchronous_grid_of_Continenta...