I think you are underestimating how long one and a half minute really is, and for that matter overestimating how much they are going to attempt showing in the cinematic. Most likely the cinematic will begin with Anduin falling over, then something with Uther and Arthas, then done.
Destroying Kingsmourne would take 10 seconds at most, and that is if it even needs to be destroyed at all, or for that matter if the cinematic begins with the sword destroyed.