I kind of don't see the benefit, as it's still going to use only the window, input and platform integrations features, while SDL3 has a whole software renderer built-in and much more.
Does this mean SDL3 has a built in 3d renderer or it's the modern meaning where anything is a serverless renderer or something like that?
SDL3 added a way to tell SDL to render vertex-based geometry as well: https://wiki.libsdl.org/SDL3/SDL_RenderGeometry. Those vertexes can have 3D positions, so that counts as a 3D renderer I guess. Though of course the GPU is doing the heavy lifting; SDL is just exposing a little bit more of the GPU's functionality. It's quite useful for some 2D applications as well. I remember reading about how it could enable more efficient drawing of 2D plots or cleaner ImGUI integration, though I don't remember details.
Genuine 3D support is available in SDL3 via the GPU types and functions: https://wiki.libsdl.org/SDL3/CategoryGPU
As long as you can draw 2D triangles there's nothing stopping you from drawing a "3D" scene with a software transform pipeline. Depth sorting gets fun without a z-buffer though.