If that's what he meant, that it's really low. For comparison a Tempurpedic is 5.3 lbs. per civic foot for it's top layer, nearer 8 for the structural bottom layer.
http://www.brookstone.com/tempur-pedic-tempur-simplicity-mat...
47 lbs / 22.22 cu. ft = 2.1 lbs
The weight listed is probably the shipping weight with packaging, so the actual weight of the foam is less. Seems like 8 lbs would be extremely dense.