Solar cells generate DC, but this is converted to AC with power electronics: the inverter is the main one. These are all just very high wattage semiconductors, but like all semiconductors, they can be destroyed by a transient event like a lightning strike, or a high fossil-fuel power plant dropping offline. To prevent the power electronics from being destroyed when this happens, they are protected by safety systems which "trip," and isolate them from the grid.

There are loads of designs for grid isolation, but most are big circuit breakers, and break the connection by rotating the conductor out of contact, powered by gravity or a spring.

So I think you're basically right, and right to be confused. The solar connection of the risk isn't really clear. But as far as I can tell, they're concerned about a positive feedback loop, where a thermal plant dropping offline will cause a solar plant to trip, which might cause other solar plants to trip. Perhaps solar plants' power electronics are more delicate, and more likely to trip, than thermal plants' switchyard gear. Again, it's not totally clear.