I kept trying to give a simple answer to this, technically the answer is a straight forward Yes, but I think its more complicated than that.

That rule was implemented to reduce the risk of a LiPo fire reaching the floor of a wooden arena, such as the Robots Live and Roaming Robots arenas, for HW's and FW's. (I believe that is correct, I may be wrong)
In beetleweights its less of an issue as 2 arenas already have thick metal floors, Stu and Dave's arenas, and ours has 1in thick wood which is treated to be self extinguishing (its old school staging and thus was required for heath and safety)

However, the main issue now isn't to do with it reaching the floor but the protection required on the sides to stop the big horizontal spinners such as Lynx and Spin Cycle reaching the LiPo by cutting though your armour and reaching it that way, rather than from the bottom up.

Given Lynx has been able to cut through 15mm of HDPE given the right hit, I would say 2mm on the bottom (if the robots base isn't that thick already) 2mm on the top and then 2mm on the sides with an additional 10mm of HDPE and probably an air gap too.

All that said though, if a robot has reached your LiPo in the first place a LiPo fire will probably be the least of your worries... your robot may well be spread far and wide across the arena by that point.