Too early to tell especially if inventory is not zeroed out. Remember, UA holds a good portion of seats in the back unassigned until day of departure for groups wanting to seat together.

Right now, 43A is open. Since the flight is a direct via HKG, you need to look at SFO-HKG as well. 43A and 47A is open if you look at SFO-HKG.

Inventory for SFO-HKG UA 805
F1 C0 D0 Y9 B9 M9 H9 Q9 V9 W0 S0 T0 K9 L9 A0

Inventory for SFO-SIN UA 805
F1 C6 D0 Y9 B9 M9 H9 Q9 V0 W0 S0 T0 K9 L9 A0

So, for someone on SFO-HKG, they might think SFO-HKG is sold out in C but in reality, at least 6 seats in C are left.