It looks like their service only works on MacOS, which means they're probably just using the local iMessage db like others have done. They're not integrating with the API or providing access for other platforms, it's a totally different (and limited) solution.
If you have a Mac sitting around, airmessage.org has made this work cross platform for a while now.
If you have a Mac sitting around, airmessage.org has made this work cross platform for a while now.