It's really no different than any auto-rebook where the pax sprints between two different gates and actually does make the original connex despite UA having decided it was impossible. Happened to me at ORD last month -- made an 8 minute connection at ORD (yes, from reaching the blocks on the incoming to pushback on the connex was 8 minutes!), while email showed me rebooked 20 hours later. Although my original flight was full, my seat assignment remained valid. I'm guessing the system wisely double books you in this situation, and does not offload/cancel you from the original trip until they close that flight and pushback. (Of course, the GA on the original connection may manually give away your seat, but that's a different matter).