I personally believe, that QT's official website is purposely vague on that topic. I contacted the sales team multiple times on behalf of different companies and the answer I got was always an interpretation of:
"To be sure you, you better by a license"
Understandable, The Qt Company is a company after all.
However as long as you yourself are not a certified lawyer, I would suggest contacting one to handle the issue for you.
If you have to buy a license, the cost of the lawyer won't change much in your expenses. If you don't need a license, It's a minor cost to bear, but at least you can be legally sure.
Many of them get confused on what Qt is, and many think that we are using Qt even though it is actually intended for something called QML and not C++
You can redirect them to Qt's wikipedia page, where it's written:
Qt uses standard C++ with extensions including signals and slots that simplify handling of events, and this helps in development of both GUI and server applications which receive their own set of event information and should process them accordingly. Qt supports many compilers, including the GCC C++ compiler and the Visual Studio suite.
and continues on to explain what QML is:
Yes, certainly good to discuss here too, but to solve the problem the Development list is the place.
Qt3D isn't complete, and as you say the documentation isn't there yet unfortunately. And as said, I believe the bigger target is to have a higher abstraction level api to 3D. Right now it is too close to OpenGL, requiring some understanding of that too.
Well, Qt is one of the most popular, if not the most popular, cross-platform GUI framework. The whole K Desktop Environment, which BTW is also quite popular at least in Europe, is build upon it. KDE comes with many applications, like utilities and games. So all in all it's no wonder that Desktop Linux distributions provide a lot of packages that depend on our cute framework.
why everything with qt in its name takes ages to compile
Can't say much about specific applications, but building the Qt libraries themselves doesn't take ages. Just did it yesterday, on Linux, took under 14 minutes.