I suspect that the seats were there, but they were blocked. When there is massive IRROPS such as the DEN hailstorm, the first thing UA did was to block seats to acommodate misconnects.

This is when having a corporate TA with 24/7 backup is invaluable. Costs a fortune, but if I call them, they will find me the alternatives, locate the seats and send me to a gate to obtain a BP.