Nothing stops an OS from providing a low contrast preference, Android's setting is even setup at the API level to support this (-1 to 1 value range). It's just that no OS does this right now, so less isn't usable on its own.
The bit where the two features do have an overlap is the custom value which is conceptually tied to forced colors mode.
The fact forced-colors mode can change the prefers-contrast value isn't unique either, it also changes the prefers-color-scheme value too.