You are of course correct. But getting into the details behind MVNOs and roaming is a much more complicated topic than I'd want to type into a small comment box, and not really relevant to the issue at hand. :)
My point is just that CMAS-style cell broadcasts need to be repeated by each cell site, and cell sites aren't universally shared by all carriers.
It appears the problem here was at a higher level, the signalling fabric that transports the data for call setup, teardown, roaming and SMS, and to complicate matters even further apparently the 4G network is where the problem originated but it had effects far outside that. That's also why the whole thing appeared so patchy from outside.
"All devices in cell" is still accurate. Each cell belongs to a single provider.
It's up to the network backend to make sure all providers get the emergency message at the same time to forward.