I worked on a similar project for a while (the poorly-named Phlisped), and I came to many of the same conclusions as these guys did. A few notes:

I'm not sure reducing keystrokes is a useful measure. I can't think of a better one, though, so at least it's something quantifiable.

Pure tree editing, with no text editor to fall back to, is important. I didn't even use textual output, while it sounds like these guys did. They also used javascript, which is interesting since it has so much syntax. Lisp seems like a natural choice to me since the tree structure is readily apparent.

Dogfooding is critical, and it's very natural for a project like this. When I got to that point, I learned so much about what was really important in an editor. Like macros -- those helped so much.

Keyboard-based editing of trees is possible, but it takes significant effort to make it feel decently natural.

Collaboration is definitely the most important thing people in this area need to learn. There are dozens of projects similar to this one, and they're generally created by solo programmers (like I did) or by pairs (like Prune). Tree-based editing will never take off until a group of like-minded programmers gets together and makes it happen. I've been pulled away into an unrelated project, but if I were to come back to this, my first move would be to get some collaborators.