How do you figure? If it’s just from the interface, they could be using a fork.
The screenshots looks like vanilla Roundcube to me, and most (all?) of the features advertised are available in Roundcube itself or an included plugin.
That said, a hosted version of Roundcube is not necessarily a bad thing, but I do wish they'd put their own spin on it and add some unique features.
What's interesting is that it specifically looks like pre 1.4.0 Roundcube with the Larry skin, and not the shinier Elastic skin.