Why is that interesting? Emacs uses a lisp; does that mean anything about emacs or lisp? (Edit: I'm being serious, I guess I don't see why Java being used to write Java editors is a big deal.)
Could you explain why? Last time I used Java (years ago), Swing was supposedly replacing AWT. Swing seemed very large (certainly, judging by the page-count of all the Swing books on bookstore shelves), and required a lot of subclassing to use.
When I tried it out, it was slow, to be sure, but it was also relatively easy to use. The layout managers worked well for my simple apps. I also liked the "Metal" theme, though others complained that Metal was unreservedly ugly.
Like everything else Java, it just does everything in the most elaborate, baroque and top-heavy way it can. There are so many, so much better ways to build GUIs out there. That's what I meant in my comment about Java editors; you need an enormous amount of tool support to do anything useful in Java because of all the boilerplate you need. Tk on the other hand is an absolute pleasure to use, and I spend almost no time on my GUIs now, I can concentrate on my real code.
Nobody likes writing Java, but enough people respect the JVM that we have Groovy, Scala, and Clojure. Why couldn't the same thing be true of Swing-the-API vs. Swing-the-underlying-UI-toolkit?