I think there is a decent chance Kong is a holy knight
Most old men are past their prime. There’s just no reason to keep a geezer as the commander in chief of the WG. The one exception to this seems to be the Gorosei, who are strong as a result of their contract with Imu.
Kong is a supreme bootlicker. He became admiral, directly serving under the CDs. Then fleet admiral. Now finally commander in chief, directly working in Mariejois. Would make sense that at some point, he just got adopted into the CD families(maybe via marriage) and became a HK. After all, I’m sure any admiral is a considerably better fighter than the average HK like Sommers.
I think Kong, having HK immortality, could be a fight for one of the legends. Maybe Garp and/or Sengoku