At the time, Nokia had even ported Qt to Symbian, which is a much more limited platform than iOs or Android. I don’t think there is any real impediment to using QtWidgets on mobile platforms.
Edit: to clarify, I mean the original Qt C++ toolkit, not the Python bindings (pyqt/pyside); the bindings only ever worked on Maemo, if i remember correctly.
Kind of.
First they had to create PIPS, a POSIX environment for Symbian, because Symbian wasn't POSIX and the native language was a C++ dialect known as just Symbian C++.
Secondly, the Qt based apps weren't pure Qt, you needed to have a couple of #ifdefs sprinkled around, calling Symbian APIs directly, even for a plain hello world.
Maemo was basically a full Linux though. It was closer to desktop squeezed into a mobile screen than a mobile platform, so lots of things could be compiled and "just work".