Why? He's right. If fixed were more agile, then trials and BMX stunt riders would use fixed, as they require the most agility of any discipline. Fixed provides more direct feedback from the rear wheel to the cranks (and may feel more agile on flat ground), but at a substantial cost in the rider's ability to apply force when/where they want (which hoses anything in 3-d space, like MTB).

Yeah, those European fixed dancers are cool, but they glide on perfect gymnasium floors. Let's see what tricks a fixed can pull on a BMX stunt course...