It's a matter of scale. If I go outside and turn on a leaf-blower I can tell you with relatively high confidence the speed and direction of general air movement five feet out from the nose. On the other hand, if I tried to tell you the speed and direction of a dozen particles at X, Y, Z location in coordinate space, the effects of chaos (particles individually colliding, etc) would make this infeasible with current technology. Similarly, describing climate is like describing the general direction and speed of air moving out from the leaf blower, while weather is like trying to make the same prediction for small groups of particles within the system. They might sound like similar tasks, but due to chaos one is doable and the other is nearly impossible.

All that being said, with regards to this particular study at the scale of 500 years it's likely our climate models nearly completely break down, as we've yet to successfully create one without significant divergence even a few decades out (although maybe that has changed as by definition the best ones we can test against only include those made before the year ~2000)