Perhaps this problem can not be solved however (i am the honest mud) so here i go. From what i can see they did climb the wall rather fast and Gecko like. When i first saw that video i thought that they had suction abilities much like that of a gecko. To further that theory they did climb the wall with ease. As if they are used to climbing vertical structures.However it could also mean that they did indeed grab hold of the groves of the boards. However by further examining the groves on the boards and the placement of the climbers feet and hands it seems in the most case that their feet have gone on the grooves while their hands are not allways on the grooves. This may suggest that they could have some sort of suction ability on their hands but not on there feet. You could say it would be like a evolved way to climb.
Or on the other hand it could be as simple as the muds being to anxious to wait out side of the mines.