"football fields" = a unit of area
350m = in units of length
... and presumably the size of the asteroid is actually best expressed as either a volume or a mass.
See this is where you've got all confused, it's not actually a 3d asteroid but indeed a 2d one from Clive Sinclairs Spectrum (I believe the 128k+ variety), as such the calculations given are correct.
Also who bloody taught these scientists maths?! I was taught to always scale down, 4 in a million..............1 in 250,000.........prats