This is very much correct. When you have 40+ assignments to grade it's not always easy to remember "Hey paper #37 looks a lot like #2". Likewise extremely large classes with multiple TA's means it's hard for one TA to catch someone that cheated off a student being graded by a different TA.

I don't know if I buy the 911 argument.

Restaurants existed prior to the cell phone revolution and I don't recall them being stacked with dead bodies because people couldn't call 911.

All you have to do is yell "This guy's having a heart attack, call 911." Someone goes to a land-line calls 911.