I've built a number of robots out of Lego Technic pieces and the only way to keep them together was often to glue pieces together. There was a MIT interstitial class (6.270) that was pretty influential in the hobby robots scene during the late 80s and one of the fallouts of that was a great building techniques guide that was notes on how to make things like power plants and load supporting beams.
That said, supporting the weight of a person and a car in the air across the distance between the front and read axles would have seemed pretty impossible to me. Now if you looked underneath and there was a row of wheels keeping the middle of the car supported that would not have surprised me. And then a top speed of 20km/h ? That is almost 20' or 6m/second, that is fast for even a small Lego car much less a huge one.