Ironically aside from Robin (and she has a history with him), one would assume Straw Hats didn't know shit about Aokiji or the admirals in general as back when he was introduced Oda had Robin info dump to the the crew who they are and where they stand in the marine and WG hierarchy, don't mean their existence is a mystery.
HKs only take care of business that directly concern the WG/celestial dragons unlike the marine that act as the world police, suggesting that the average joe likely knows next to nothing about them but does it mean their existence is concealed? No reason to believe so, they aren't spies that are mean to infitrate like the CP9 but warriors/knights who conquer territories for the WG.
In this very chapter Sommers announce to the entirety of Elbaf that he's a holy knight when he could've just said he came on the behalf of the WG without disclosing the group he came from (if again that was mean to be something secret).
Usually when something is mean to be secret Oda is making it clear, I don't get that vibe from HKs, they've been namedropped casually since the end of Wano.
Robin and Jinbe not knowing at all about the HKs is a big leap too, it's based on them not immediately recognizing Sommers and Gunko which is a flimsy evidence. Even though they knew about admirals and even faced them before, straw hats didn't recognize who Fujitora was until they were told who he was.