The floor is there to allow the Mechitzos. Gedi’im Bok’im ruins the Mechitza.
Aggadaicly, the floor fits right in, since the Succah is a Zecher to the Ananei Hakavod which were on six sides.
Over here the issue is more than using a stolen floor (which can’t be an issue because of Dal Ashteros Karnaim). The issue is the spot, which means the whole Chalal Hasuccah. On the other hand, being illegal to do something probably doesn’t translate into Gazul.