The difference is that its up to you to provide the backend, so presumably you will be using whatever you're application is already using, rather than pulling in a new dependency just for Nuklear to work.
Its still a dependency in my books, but its not the same as saying "this requires DirectX" or whatever: it requires a backend, but you can give it any backend you want, so ideally wouldn't pull in anything you aren't already using.
For these imgui toolkit, it is just a few lines of code if you know what you are doing. For the library authors, it is quite a bit of work to maintain different environments and CIs to test all these backends though.
Its still a dependency in my books, but its not the same as saying "this requires DirectX" or whatever: it requires a backend, but you can give it any backend you want, so ideally wouldn't pull in anything you aren't already using.