In the last two lectures (Potpourri, Q&A), they actually have a grad student that uses Emacs go over it as well. The TA that was in charge of the editor section

In the last two lectures (Potpourri, Q&A), they actually have a grad student that uses Emacs go over it as well. The TA that was in charge of the editor section was just a Vim user and he wanted to show what kind of things are possible with a highly configurable editor like Vim/Emacs, so he naturally was going to use what he was most comfortable with. He even mentions that Emacs is capable of doing pretty much everything that he goes over in his demo and that it's mostly a matter of personal preference.