My take was that the Tholians had some magic way of walking on the walls despite gravity. Much like spiders and other insects do on earth. If they had gravity generators in each floor/wall/ceiling, then you (the player) should be able to walk on those surfaces as well. Since you can't, that's clearly not the way it works.
So if we get Tholian Boffs, can we have them walk on the walls? It is their ship design after all so it doesn't make sense that they are limited in the same way that players are.