Looking at their specs they claim it generates 225 pounds of pull off force, estimating the vacuum chamber to be roughly 12" x 12", that means there is 144 in^2 of area, this would require about 1.56 psi vacuum, doesn't seem too far fetched to me. I believe they are powering this device externally so they have plenty of power available.
I read somewhere that this device is capable of driving over protrusions such as riveted seams, not sure what the exact spec on this is but the thick and wide foam treads/rollers allow the robot to create a seal even with non-smooth surfaces.
I few years back I saw a story on a guy that climbed the outside of a building using a couple of vacuum cleaners and some suction plates he made.