SDL doesn’t currently have any implementation of high DPI scaling support on Windows - anything you observe is done entirely by the OS without SDL’s knowledge.
It does have high DPI scaling implementations on macOS, iOS, and Wayland. There’s a bugzilla post which has a history of the current state of development of high DPI scaling on Windows, here: https://bugzilla.libsdl.org/show_bug.cgi?id=3281