> but I thought they were incredibly hardened, with backups and contingencies in depth

Some are harder than others, and some have random flaws which nobody can really predict.

Spain seems in the transition to renewables, so it's possible that they have some flaws because they are still in the process, or because it's something which never happened before and is unknown territory. Also, Spain had some economic problems in the last decade, maybe someone build to cheap or was even cheating somewhere.

> Are the grids at this scale really this brittle? Would there be a death toll from this?

Hospitals should have backup-systems. Traffic should be able to stop in time. I guess the most problematic parts are people stuck in elevators and other spaces which only open electrical, as also the loss of cellular phone-connections for calling helpers.

Hardening focuses first on not damaging equipment and second on providing energy. If things go wrong quickly enough you don't have time to react, because after a power plant disconnects you get sudden bumps in load that can trigger a chain failure near the original point. The last time it happened in Europe was in 2003, which isn't too shoddy.

What Spain's PM is saying (and is being reported by Spanish newspapers), 15 gigawatts of energy production went down all in 5 seconds. Hardening to tolerate that much of a change, that fast is a more extreme event than a grid the size of Spain's preps for.