I'm not 100% sure, but I think it might be even better than that. It is possible you can book your companion as long as the oversell limit hasn't been reached. When trying to gauge how full a flight is I sometimes compare companion availability to purchase availability. I've yet to find a case where they differ. Perhaps, however, none of my searches had a sell limit over 100%. I therefore don't yet have assurance my data means anything, but my suspicion is that any allowable overbooking is also available to a companion.