Even if it was, when every flight from every co-terminal is sold-out, UA just plain can't honor the co-terminal rule. I am sure UA knows that every plane should go out at 100% because they're trying to pack multiple flight's worth of people on the flight that can get out.

UA, like any other airline, tries and sells as many walk-up Y fares as they can. That is why they offer VDB tickets and vouchers because they make more money on the walk-up fare then they do on the voucher/ticket. So as abramon was willing to pay the listed one-way MCO-IAD fare, they were willing to sell him the seat and, if necessary, IDB someone on an S or T fare. Or, since the exit row was open, it is possible another passenger with changeable/refundable tickets gave up their seat, which made it available to sell.