Probably a pessimistic estimate. I did some explosive dispersion experiments back in the late 1990s using a hot cell and some decayed fuel plates from a research reactor. The most dispersion we got was in the range of 50 sq. m. or so. The stuff just doesn't pulverize and disperse very readily.
Most portable instruments have quite weak sources (which is why they are portable). Those that have stronger sources are more heavily shielded, making it difficult to access the material and more difficult to disperse. Once you do have dispersion, even a little, the intensity of the source is reduced by the change in geometry. I'd be more worried about blast effects than contamination.
It has Long been my opinion, based on 25+ yrs in the Nuclear Operations, Rad Safety, Irradiated Waste Processing fields that a ‘dirty-bomb’ is destructive only by instilling panic in a nuclear-skittish public. Yes, there will need to be a decon effort. But that boils down, realistically, to nothing more than a pain in the rear to get done.
The real efforts at stopping an attack really should be focused on those that are destructive, rather than those that are a pain.