There is a general ignorance amoung cyclists as to the sheer variety of rear brakes, largely because of the predominace of a couple of kinds of brakes. Certainly a fixie is a very effective, if primitive, brake, and works well. The most effective rear brake, by far, I've ever had was a hub (drum) brake on the rear of an old beater/commuter. It would stop you fast, no matter what the conditions. Truly a phenomenal thing, and I would recommend it strongly to any newbie in traffic. Some cyclists would not recognize it as a brake, however, so accustomed are they to caliper and disc brakes.

I doubt the passing rider understood the mechanics of the fixed gear. Too used to a freewheel.