In one sense, you are right. A fixed gear is no more than a single speed bike that doesn't coast. But that one difference is so great that it absolutely changes the entire experience, especially when out of the saddle or riding aggressively. And especially when descending.

On a freewheel bike if your foot comes off, you can just stop the cranks and put the foot back on the pedal. On a FG, the pedals keep going around. If this happens during a sprint, I think most riders will simply crash. No wait, I take that back. First they'll be flung hard forward, their reproductive bits onto the top tube and their teeth on to the handlebars, and then they'll crash.