I would suggest you check their calculations, first of all, Then maybe have them reperform the test in a parking lot. I am familiar with Chicgo area, I don’t think there are any neighborhoods in northern Il. that would have a Hill steep enough to get a kid on a mountain bike that fast. Much less a group of them. MAYBE in one of those Chicgo winter storms where the 40 MPH winds and they had about a mile and half to build speed. But then I doubt it.
30 to 38 MPH is low level pro racing on a road bike. I have acheived 35 MPH on a flat road on a road bike many times. NEVER have I riden that speed on flat surface on a Mountain bike.