Im not a expert on the topic, but "2D skybox" texture display the skybox texture you currently have assigned to the map, while "Skybox" texture displays an 3D skybox.
I think the problem is that the map is so open, which I don't really want to get rid of because flying around as demoman is just so fun.
2D skyboxes are just a cube you're inside of, 3D skyboxes have actual geometry and perspective and are basically just like the map you're in but rendered separately.
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