They have disco lighting effects and a fan blowing on the yeti to allow the fur to move... it's a very well done effect and people still find it believable.
No excuse for them not to fix it, but if you didn't know about the Yeti being broken, you would think it did move. While the Yeti is impressive, the ride is more than great without it. Kong, especially now, can't run if the animatronic doesn't work. The payoff is just not there.