Yes, ptrace can be globally disabled by a file in /proc somewhere, I believe. I forget the details.

But even with access, this is awfully hard to exploit. You need a precisely timed kill, followed by a precisely timed preemption. That's really (really) hard on a multi-cpu machine, and it's worth pointing out that the exerciser case in the bug has to patch in a sleep for the exploit to work. It's not clear if they have a real world exploit running, though of course the race is real.