I do think kids would be significantly better during an actual safety event.
There are so many drllls—no one takes them seriously. When my office has a fire drill people just ignore it or go out for coffee. And those are adults.
The times there have been actual active shooter events in the schools — such as one at my kids HS where there was a kid with a gun on campus and they locked down the classrooms—they kids get quiet pretty fast. These kids have grown up in a world of school shooters and climate disasters so they know how quickly stuff can go wrong.

A kid got stabbed during a drill at Alexandria City High School so at least you don’t have that.

I'm in a high end private school and our kids are HORRIBLE during safety drills. There is no school wide expectation of silence and during one drill, kids were screaming in the stairwells. When I went to admin to try and address this school wide, nothing was done, no sense of urgency to change the behavior was shared. In a true emergency, kids at my school will likely die because they refuse to listen.
I will tell you that during public school safety drills I've been a part of, kids are silent. We did not experience kids leaving after attendance was taken like the OP describes, but I do know schools like that. It's all screwed up like everything else.

the middle schoolers at my school could not handle any type of real evacuation from our school campus as well as those ES students.

This it worries me because we have set very low standards for students following directions, even during safety drills. During our evacuation drill, sixteen kids (out of 1000) sneaked off once attendance was taken. Some went home. Others went into the neighborhood and nearby park and woods. A few went to local fast food places.

I feel like I have not received any information about how to keep kids together, safe, and calm in a real emergency evacuation. My last experience was as a camp counselor twenty some years ago.