I'm sorry, but I don't think that assertion makes sense.
You can run any app given you have the supporting libs under any desktop. You prolly know that too. Apt-get, yum, yast, up2date w.h.y. will pull in all that you need for any app.
True, you've always been able to run Qt applications in any destkop.
As Mark puts it in his blog Qt apps on Ubuntu
System settings and prefs, however, have long been a cause of friction between Qt and Gtk. Integration with system settings and preferences is critical to the sense of an application “belonging” on the system. It affects the ability to manage that application using the same tools one uses to manage all the other applications, and the sorts of settings-and-preference experience that users can have with the app. This has traditionally been a problem with Qt / KDE applications on Ubuntu, because Gtk apps all use a centrally-manageable preferences store, and KDE apps do things differently.
To address this, Canonical is driving the development of dconf bindings for Qt, so that it is possible to write a Qt app that uses the same settings framework as everything else in Ubuntu.
While he doesn't say that this is the purpose of Unity directly, it can be (by me at least) infered by
The decision to be open to Qt is in no way a criticism of GNOME. It’s a celebration of free software’s diversity and complexity. Those values of ease of use and integration remain shared values with GNOME, and a great basis for collaboration with GNOME developers and project members. Perhaps GNOME itself will embrace Qt, perhaps not, but if it does then our willingness to blaze this trail would be a contribution in leadership.