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This is not true. Look into Zigbee binding, it works when the coordinator is offline.

I'm using that as a backup solution in my house. All lights are zigbee, permanently powered, normally most are controlled automatically by presence sensors. But I have a couple of Zigbee remotes in a box that are bound directly to the lights to use in case my home automation system is down.

I'd modify my comment if I could. Bound some remotes to some hue bulbs, proving myself wrong.

Not all, IKEA Trådfri works fully hub-less by default.

TIL I can bind my remotes to my Hue bulbs, and they work without the coordinator.

So how do the Hue light switches work?

Hue light switches (well, the ones I have) simply stick on the wall with no wiring.

The _real_ light switch is permanently on, and the _hue_ light switch sends a beacon to the hub to turn the configured lights on/off (the light bulb socket is always-on)

No, some switchers and dimmers can also control lights without a hub present. Some products come as a paired bulb and switch, with only an optional suggestion that you later extend into the ecosystem.