In other words, the act of planning means you're better prepared for specific contingencies, so you'll hopefully be better prepared for whatever actually happens, but some improvisation will always be necessary.
The planning may be directly applicable in whole, is almost certainly applicable in part but in my opinion the main thing is that it provides a template for thinking about what things are important and which are expendable when you're operating in an environment with a greater tolerance for risk.
As I was writing this comment I learned that they were inspired by a bombing which happened in Brussels: https://www.shponline.co.uk/fire-safety-and-emergency/mascal...
One of those things you hope will never need to be used. But if it is ever needed it will be very handy.
However, a shooting is just one type of mass casualty event.
It always happens eventually in discussions like this. "I'm from <country in Europe> and I don't get why y'all are so stupid, unlike us."
The US healthcare system is worthy of critique for many things, most of all cost, but the quality of care is just as good as every other western nation. Doctors are quite skilled, just like they are in Belgium I assume. And of course hospitals plan for mass casualty events. All of them, I bet, if they have an ER.