Except the Mahouka-verse's definition of lightspeed is explicitly different from ours.
How can one universe have hypersonic humans while the other has normal ones?

Surely humans can't be that fast right? Perhaps it was the bullets that were moving slowly?
See what I'm getting at? Not to be sarcastic but that's the best I can think of in this sleep-deprived state of happiness I'm in at the moment.
Not everything that applies in one verse applies in another, same with laws or fundamentals or other big words of reality. Especially when we're given explicit evidence.
Not to say the whole thing is correct though. I'm gonna have to ask who said the words, "the speed of light is probably hundreds of times faster than the speed of sound" and is his/her words credible enough to be taken seriously, as there are those characters who have varying degrees of stupid.
I'm also questioning the validity of the translation, and whether or not it is the official one and is without errors. It's extremely varying in speeds, which to summarize the thread:
However in the present interval, the distance of sixty meters was spanned in less than two milliseconds. Which is the same as being instantaneous.
Mach 88
The beam just now was as fast as the speed of light.
SoL
The speed of light is probably about a hundred times the speed of sound.
Sub-rel