the encounter against Garp pretty much confirmed that Kuzan turned his back on the Marines.
Are you expecting Kuzan to expose himself in front of the other pirates when it comes to his true goals?
"Hey Garp, it's not what it looks like! I'm just fooling this fat neckbeard for the secret mission I've been working on!"
Ofc Kuzan has to show his persona as a former admiral who is now a pirate seeking for true freedom in order to not show suspicion.
No shit, all of them have ulterior motives but the difference is that none of the other pirates, not even Shiryu, can possibly work as a spy for a certain reason, especially since none of them are known to be good characters to begin with, unlike Kuzan.