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?
Same way as you would for any linux live usb which you boot to ram.
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.