yes, and the test course was uphill (no ones contesting that bigger is better downhill).
Thus there is a power requirement to lift the extra 800g (400 at each end) from the bottom of the hill to the top.
Say his 500m test hill was a gaining 80m in height. And he's completeing the course in 189 seconds at 9.5kn/h.
it takes 627J to lift the tires to the top of the hill, so a power of arround 3.3W. Thats for a completely smooth road test, as he points out, the continual bouncing up and down will increace this power off road.
And that is a great example of science by press release whereby you release results to the press/public before its published in a scientific journal. A bit like that "missing link" fossil in germany last month. Theres more money to be made from the public press than they would ever get in research grants resulting from scientific publication.