I’ve been learning this for some time now and am customizing it quite a bit for my work needs. I’m just thinking out loud here - since development has slowed down, or maybe Jon Skinner gets bored with this project, will it be open-sourced? I’d hate to be in a situation where I’d have to learn yet another editor.

Eh whut? What wicked rumour you’re spreading here?
Not having a release ready every each other day does not mean a slow down or lack of interest. To prove: take a look at the Scintilla / SciTE project. Sometimes there are new releases every month, and then half a year nothing. And Scintilla / Scite is ‘only’ an editor framework and an accompanying demonstration of functions.

Development has not slowed down. There have been several releases recently which have made huge changes to syntaxes.