Who are the fastest readers in the world? 9/11 victims. They went through 77 stories in 7 seconds
Where did you hear that joke about 911?
It's an adage founded by Nietzsche.
Terrorist attacks like 911 are traumatizing events for everybody and it can be triggering to even talk to them about it much less joke about it. Making light about it can be hurtful, just like you would be hurt if people made a joke about something that caused you pain.