An adult red dragon is within melee range of a fighter with the Sentinel feat.
In turn, the dragon moves away and the fighter seizes an opportunity, reducing the dragon's speed to 0.
From the Sentinel exploit (PHB, page 169):
When you hit a creature with an attack of opportunity, the creature's speed becomes 0 for the rest of the turn.
The fighter then takes his turn. At the end of the combatant's turn, the dragon takes on the legendary Wing Attack action.
Wing attack (costs 2 actions)
The dragon flaps its wings. Each creature within 10 feet of the dragon must make a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked out. The dragon can then fly up to half its flight speed.
Am I correct in assuming that the dragon could not move because its speed would be 0?