K, how is climbing "way too complex to put in"? Maybe you walk towards something deemed "climbable", hit the jump button, and, you climb.
Sure, they don't need to test it. They don't need to make sure it doesn't completely unbalance and break things. It won't take any time at all to go through the entire gameworld and check every single surface that might be climbable and then check if adding that property would ruin any of the quests, etc.
This is just for climbing. The fact is, designing a game with free running/parkour/Matrix/breakdance movement means specifically designing it to include that sort of functionality, not just "throwing it in." That's how games get broken and crap.