I don't believe this is a true statement - You can unplug the rejector and it will hold it's memory. I used to carry unplugged spares in my service truck all the time. Plug them into a working mech and away you go.
if you unplug the electronic coin detection (slug rejector) with the power on, it will scramble the memory, it's cautionary statement, if you buy a MARS 6800 or similar mech, from someone who did this, or worse, if they mixed rejectors between mechs. but usually retuning the mech's will fix the problem.