If I recall correctly, the damage mechanism in cases like this is not simply wind blast. Upon encountering the ground, the downwash flow has nowhere to go except sideways and expands as a high-velocity sheet, which results in lower-than-ambient pressure because Bernoulli, and since there is air under the matting there is a pressure differential, and it doesn't take a lot of delta-PSI over hundreds of square feet of matting to generate useful lift.
I recall John Fozard saying that this phenomenon was not fully understood before early P.1127 tests until a cast-iron manhole cover became airborne during a VTO event, to the general discomfiture of those present.