The Horde took a bigger hit throughout “Battle for Azeroth” leaving them weaker and with the Alliance attack on Dazar’alor without a fleet (actual source being a statement coming from Nathanos himself). During the Nazjatar intro we only see a handful of Kul Tiran warships, seemingly meaning that the bulk of the fleet wasn’t in pursuit but either on standby or still returning from the attack on Dazar’alor.
(Ironically that means that Arctur was right during the initial discussions about the cinematic. I cannot accept this, but I have to. Araphant help me.)
Seriously, you argue about something that has never played any role at all. Fleets, armies, airships, manpower, weapons, tanks and everything else will always be at hand when the plot needs it.