The Thief's Climb works much like the Knight's Ancient Map.
It allows passage through/over the wall, but does not provide any bonus movement on its own - all of that has to come from the Thief's movement stat.
Reading the original rules for the Thief, the last paragraph of Climb states:
Original Rules wrote:
Using this action does not reduce your movement cost to make these moves.
Ah, I see the misunderstanding now -- when I read it the first time I interpreted that line to mean that movement points were not reduced by climb. A better phrasing might be:
Proposed Change wrote:
Using this action still costs one movement.