Emacs is most of the way already. It doesn't have an interactive code display thing like this (as far as I know), but most everything else either already works or works similarly.

For example, getting documentation is just as easy. (Actually, I don't know if you can search by docstring, but otherwise it's the same.) You can also get the same experience as having functions rather than files open by having a lot of little "windows" (in the Emacs sense) with a function in each. Since you can have multiple "windows" open on a single buffer, it can work for functions in different parts of the same file as well.

In short: Emacs has all the building blocks you need (as always) and some of the features are already easy to replicate. Building something like this on top of Emacs just makes sense.

I was thinking the same thing. Many of those ideas would be great emacs modes. In fact during the first minute of the video I was going to ask how he skinned emacs to be so pretty :)