Not saying these aren't important, but you can implement these features once you've got a nice API working. You can't easily retrofit a nice API into an existing project with lots of features. So maybe they chose the right way.

It claims to be a GUI library in modern C++, not (necessarily) a modern GUI library in C++.

In all fairness, it does not say that the GUI library itself is modern in that sense, but that it uses modern C++.

Well for something like a gui for a Linux lv2 audio plugin this is pretty quick to learn and much smaller than qt