It appears their client is open source, but I agree that they are not open source in the expected way (i.e., can't self-host): http://blog.filerock.com/2012/12/were-going-open-source/
The client code is GPLv3, so it could be a building block for other projects if they want to use it with a different server.
Also, it isn't a Dropbox clone (you wish!).
But yes, it's not an "open source <something>", it's a "<something> with source available". It's great from a security standpoint, but "open source" implies that all the components are open-source and that you can self-host, and referring to it as "open source <something>" is disingenuous.
That said, this is just the HN post's title. It's not described like that on your site, so that part is good.