Actually that's often mistankenly thought, and I did that too. The Qt Components are open source, and the MeeGo Touch Framework (https://meego.gitorious.org/meegotouch) that is used in Harmattan on N9 is open source. The modified mcompositor and swipe gesture are proprietary, as are the graphics.
Meego touch framework is only in maintenance mode now and kind of deprecated (although that was from MeeGo side that now no longer exists), but elsewhere the Wayland compositors that can be based on Qt Compositor framework can manage similar level of performance as the custom mcompositor, and Qt Components (https://qt.gitorious.org/qt-components) is continued to be developed.
It's more about missing media coverage that makes it easy to assume N9's UI has hidden proprietary magic. One can also study their X.org and Qt sources among else to see if there are some optimizations.
They just finally had mature technology (Linux + Qt) and resources and learnings to pull off a smartphone like Nokia N9. Hopefully some other vendors see the potential here, and not to forget the rumored Nokia's Qt for next billion either.