Before watching the programme I didn't know the house had 2 floors, quite impressive! But the whole 2nd floor was supported by the wooden structure.
It all goes back to the problems of building a solid beam out of LEGO which they kept failing at, and even at the end they still didn't want to risk placing a person on them supporting LEGO beams without safety measures.. which is fair!
They also skipped the detail about how exactly the stairs were built, can only come to the conclusion that it was also supported by the wooden structure.
Is still impressive what they achieved but hmm less so impressive!