Having source on GitHub is not the only definition of "open source." The generally accepted definition is that the source is available (you may have to ask for it), it might be free (you might be charged for media, though that is less of concern here in the future), and the recipient can modify and redistribute the source.

It might be more appropriate if the site had text like "if you are interested in the source code, please email us." As it stands, it does seem like they are selectively releasing the source (and this is an assumption, right, since the site doesn't say "request an invite and get the source?").