Further down the article ( http://www.ridedrive.co.uk/tipoffs03b.htm ) more about his thinking:
The heavier the vehicle the more momentum it has, but the heavier the vehicle the greater the down-force there is transmitted through the tyres to the road giving a better braking effect. The extra weight is pushing down on the road more, but it is also pushing forward by more. Put extra wheels on to make more surface area touch the ground and the weight is spread over a larger area and becomes less at any given pin-point cancelling out any advantage that you think you would have had. Take a 28-tonne 4-wheeled vehicle and a 28-tonne solid block of tyre rubber and each will slide for the same distance from the same speed on the same coefficient of friction road surface.