If security wants to cock block 911, they can call them back and direct the response to a specific gate, or cancel the call, or something.
No, they can't. If the ambulance service is on its game, they likely are at the gate or very near by the time someone thinks "gee, maybe 911 won't know where 'the brown trailer off of that one alley next to where the old greenhouse was' is and I should call security. You know, the guys we can reach by dialling the extension posted on the "Emergency? Call xxxx" sticker on every single phone in the complex or keying up the emergency channel programmed into every plant radio." Oh, and the reporting party hasn't been anywhere near the gate in hours and has no idea the gate they just sent the ambulance to is congested by truck traffic, and they now have to redirect. Or a car carrying product from an entirely different plant spilled near an access road and the clean-up crew has traffic blocked, making what would usually be the most efficient route the absolute worst at this very moment.
But absolutely, when your co-worker has collapsed from chest pains, the best plan is to initiate a half-assed response contrary to all of the plant policies and training that everyone completes, that takes incredibly much more time because those security guys are just dicks who HAVE to know everything.
Those policies exist for a reason. They weren't written to shaft the little guy, they're there to ensure we can get assistance to him/her when needed. It's not really in the company's best interest to have people, you know, dying at work. I thought the "don't call 911" mandate was ridiculous too when I started that job. Didn't take long to see why it's necessary.