Yes, but that actually makes it harder to start up.
To synchronize the isolated grids, they all need to operate with an exact match of supply and demand. Any grid with an oversupply will run fast, any grid with an undersupply will run slow. When it comes to connecting, the technical source-of-truth doesn't matter: you just need to ensure that there will be a near-zero flow the moment the two are connected - which means both sides must individually be balanced.
And remember: if you are operating a tiny subgrid you have very little control over the load (even a single factory starting up can have a significant impact), and your control over the supply is extremely sluggish. Matching them up can take days, during which each individual subgrid has very little redundancy.
On the other hand, the interconnect essentially acts as a huge buffer. Compared to the small grid being connected, it essentially has infinite source and sink capacity. For practical purposes, it is operating at a fixed speed - any change is averaged out over the entire grid. This makes it way easier to connect an individual power plant (it just has to operate at near-zero load itself, move to meet a fixed frequency target [which is easy because there is no load to resist this change], and after connection take on load as desired) and to reconnect additional load (compared to the whole grid, a city being connected is a rounding error).
To synchronize the isolated grids, they all need to operate with an exact match of supply and demand. Any grid with an oversupply will run fast, any grid with an undersupply will run slow. When it comes to connecting, the technical source-of-truth doesn't matter: you just need to ensure that there will be a near-zero flow the moment the two are connected - which means both sides must individually be balanced.
And remember: if you are operating a tiny subgrid you have very little control over the load (even a single factory starting up can have a significant impact), and your control over the supply is extremely sluggish. Matching them up can take days, during which each individual subgrid has very little redundancy.
On the other hand, the interconnect essentially acts as a huge buffer. Compared to the small grid being connected, it essentially has infinite source and sink capacity. For practical purposes, it is operating at a fixed speed - any change is averaged out over the entire grid. This makes it way easier to connect an individual power plant (it just has to operate at near-zero load itself, move to meet a fixed frequency target [which is easy because there is no load to resist this change], and after connection take on load as desired) and to reconnect additional load (compared to the whole grid, a city being connected is a rounding error).