"tested on their ability to discriminate tone"

That's not what perfect pitch is. Lots of people can discriminate tone well but do not have perfect pitch.

Perfect pitch is the ability to listen to a note and state, "That's a D, that's B, that's a G#, (or whatever the note happens to be.)   

True story: When I was a college student, I had a summer job in a park, and one of my fellow workers was a music student. One day we caught the bus to go home, and some part of the bus was emitting a worrisome, constant, high-pitched squeal. I said, "I wonder what that is." And she said, "It's D."

That's perfect pitch.