Surely to be the fastest (hu)man alive (or is it merely fastest of all male humans?) you would have to average out as the fastest of every single process that time could be measured for, which would probably take more than a lifetime to measure, and therefore could not be measured.
One thing is for certain though: Matt is certainly not “definately” the fastest man alive, as there is no such word spelt in that way. He could “definitely” be the fastest man in the world: that is what should be argued here.