It does seem like an odd choice given that Creative Commons specifically recommends against it[0]. That alone would make me unlikely to use this project.
"Open Source" means the source code is accessible, usually to the entire public. "Closed Source" means the source code is not accessible.
Whether the source is open or not does not limit what type of license that can be applied to it. For example, I can write a program and not release it to any other soul and give it an MIT license. Conversely I could publish code on github available to the public and apply a very strict license.