It's both. There's a cooldown for the player and there's the fact you can't ride an airship that's deployed.
So technically I guess you could have the next group not on cooldown after the 90 mins take the next airships then those previous players have to wait 60 mins instead of 30 mins. That's going to be some hard scheduling. It'll probably all be a moot point after it's been zerged to death over the 1st month but that 1st month will be a lot of drama is my guess.
Airships in use won't be unusable for 90 minutes. They will return after 20 minutes meaning that an airship can theoretically transport 3 parties within an hour.