That's the whole point!

You plug it into your project and it can be rendered on anything that can push pixels and/or triangles to the screen. Events from windowing system go in, list of triangles comes out.

This is intended to be used with OpenGL, Vulkan, D3D and other graphics environment and used in cases where integrating a "real" GUI toolkit would be more trouble than it's worth.

Other popular libs like Dear Imgui or Egui work the same way.