> I am saying that this point does not make sense - else every library which has some concept not perfectly transmitted by raw C++ code (for instance : a library function whose correct execution / lack of UB depends on the programmer calling another function before that) would be problematic.

My point does make sense, you're just missing it.

First of all, my point is that Qt is a poor developer experience--that its interface is bastardized is merely evidence. Further, even if it were not bastardized, it would still be awful because it's C++ (insane build tooling, no package management, poor error messaging, no memory safety, enormous feature matrix, etc etc etc; all of this has been covered elsewhere ad nauseum, no need to repeat those arguments here). There's a long tail of additional issues with Qt that contribute to its unpleasantness, but there's no point in diving into them here.

Secondly, your stateful library functions example doesn't fit my criticism because stateful functions are part of the C++ language spec while signals and slots are not.