That's not really correct. Harmattan is based on the "MeeGo Touch Framework", which is a very thick layer on top of Qt. MeeGo proper still includes this layer, but it's deprecated and expected to disappear eventually. The UI framework for MeeGo's future is just QML ("Qt Quick"), with a library of common components.
Basically, while MeeGo and Harmattan share roots, UI code can't be meaningfully written to run across the platforms.