Totally non-expert thought: Isn't that only true if you're total RAM cpacity is N(2), where N is the amount of RAM for the server. Due to the volatile nature of memory, without snapping, freezing, modifying, and promoting how would one envision updating a RAM-disk OS?
You only need N(2) if you're updating by downloading and switching to an entire second rootfs image. If your OS takes up 500MB and you update a single 5MB component, then you only need 505MB, and that only until the update completes and the old version is dropped and you return to 500MB.