Its been a while since I watched that episode, but that was built with a steel superstructure surrounded by lego right? The lego was only partially load-bearing?
Around 23:30 you can see that they added a wooden safety structure, which did not (normally) hold any weight. It was just there in case the Lego failed. The Lego alone actually held itself, and the occupants, just fine. The first test is at 40:30.
