They said, "MIT License."
But remember, "open source" . . .
- Does not mean that someone does not own the code. Copyright still applies.
- Does not necessarily mean that every patch you submit will be accepted.
- Does not necessarily mean that access to the source-code is "free," although in this case it is.
So, if you really want to hack another source-code editor, now you can. (I haven't seen if they included their compiler.)