By calling dlopen() in runtime? Or how exactly?

The answer is linked to by the section of the article you quoted:

> (1) If you statically link against an LGPLed library, you must also provide your application in an object (not necessarily source) format, so that a user has the opportunity to modify the library and relink the application.

And also answered directly in the section you quoted:

> as long as you provide the object files and allow users to relink your app with a different version of Qt

By linking the provided proprietary object files with Qt compiled from source.