Ya, but they're wrong. Open Source is all about the license.

EDIT: OK, I guess you need the actual source code, too. But once you have the source and an open source license, it's open source, whether it's on github or stored on a thumb drive, and no matter the size of the community around it.

Although I guess you have a point about there being other things that are important... like you might say, Android isn't _really_ open source any more because of the way that Google has gradually made the whole ecosystem dependent on proprietary stuff like Google Play Services, and because of the fact that development happens behind closed doors at google, but technically AOSP is still open source.