I wish they made their licensing clearer. Their website has nothing to say about this. There is no "LICENSE" file at the top-level of their main source repo, nor any discussion of licensing in their README.

Many of their actual source files have a copyright notice at the top including an EPL (Eclipse Public License) grant, so it looks like this is licensed under EPL. But they could make that fact clearer. (Also, I haven't actually checked every single source file, so there is some risk some of them might be under different license terms.)