I would argue exactly the other way around: Qt is stand-alone and GTK is not. If you want to write any app you need more than the UI. You actually want the application to *do* something but render a couple of widgets.
With GTK you end up hunting down a host of glib/gnome based libraries, all with slightly different peculiarities and all of them coming with little useful documentation. How is that stand-alone? With Qt you get everything in one convenient package (and are still free to leave out the parts you do not need in your binary packages).
Qt is a C++ library: Any C++ compiler can compile it on a wide range of platforms. How would that be possible if Qt was written in a weird dialect?
Of course the object model leaks into the language bindings. How could it not be? The same is true for "object-oriented" libraries written in C.
Yes, even with Qt you can not get perfect cross-platform applications. You will need to some platform-specific code in any non-trivial application. That is perfectly possible in Qt... and it still gets you at least 90% of the way! That was the other reason for switching that Dirk gave in his presentation: That GTK does *not* run properly on windows nor on Mac. He claimed that some core GTK people are actually opposing the toolkit working on those platforms and that independent teams are trying to maintain the cross-platform parts as good as they can against a hostile core team.
Subsurface was cross-platform with GTK and it looked like shit on *all* platforms incl. Linux. The Qt port looks way better -- they could finally get the UI they wanted but could not manage to implement in GTK -- and works equally well on all three target platforms. Check the demo right in the middle of the video: Dirk shows of the new UI and contrasts it with the old one in pretty gory details. So, yes, Qt is not perfect, but it is pretty good nontheless:-)