Yes these entirely fit the platform/library hard distinction model.
The platform is the small API to create windows, recieve input events and get an OpenGL/Vulkan/DirectX/Metal surface to draw to, as well as the graphics library itself, which is similarly based on primitives.
Everything above that is a library.
> Because the users of those systems don't really want a canvas they have to render everything on... rendering is hard.
Qt isn't a "system". libQt5 Core + Gui + Widgets + Qml is roughly 20MB on a Linux system I can readily check. This is way more than lean enough to ship with every app and even to download-on-first-use and cache for a web app (assuming it was ubiquitous and many websites would want to pull it from cache).
What the system gives is exactly a canvas to render stuff on.