That wasn’t Conqueror’s Haki, it was CoA. Garp’s Conqueror’s Haki was confirmed in the Vivre Card, while Aokiji’s wasn’t. So what, they didn’t have enough material to confirm Conqueror’s Haki for Aokiji?
VC go for what they know from Oda. They didn't even confirm Zoro's CoC back then.

