Normally you ask them for the object files, but those specific guys are middleware developers so they provide QtCreator configured to build with their library (to get a different Qt version, select a different Qt Kit in QtCreator settings).
Additionally, the nature of the library is that it's probably a plugin that's loaded by a regular Qt application, as a result you've got their classes available in the QML scripts. It isn't very intrusive on the practical level.
They don't say what license they use though.
