Flatpak doesn’t come with more libraries to interact with other flatpaks. It comes with libraries that the application’s flatpak you’re downloading requires. However, when installing the flatpak those libraries do not get installed if they are already on the system.

So widget-flatpak needs lib-a and lib-b. You’re system already has lib-b that flatpak is using for as another flatpak.

You install widget-flatpak. lib-a gets installed but lib-b does not because you already have it.