Also why do you call it compositor, when it actually seems to be a sway fork?

because it is called a compositor, just like sway, hyprland or any other wayland compositor. although they're so often called window managers most people consider it correct too, just know that compositor is more specific to wayland.

Another WM? What differentiates swayfx from hyprland?

swayfx is a sway fork unrelated to hyprland, since i already had a sway config and wanted some eyecandy, i decided to switch to swayfx which was easier than switching to hyprland