If he's Fleet Admiral in Roger's time. it is because it is extremely powerful, there is no debate on it.
Kong has been promoted as Commander in Chief of the Whole Army of the World Government, which is also the most powerful organization in the world which includes
Marines / Who is Superior to Fleet Admiral
Ciphol Poll / CP0
Impel Down / Superior to Magellan and Shiryu
The Former 7 Royals Shichibukai / Superior to Mihawk and Weeble
The Government's Espionage Agents
There is absolute control over all these groups of organizations.
We can ask these questions there.
Kong has a scar / mark on his left face like Garp, Dragon and Luffy. Is he part of the D Family / Clan? Mainly Monkey.D? sight that Kong and Monkey come from the primate family.
Monkey. D Kong sounds cool too lol. Some people even think he's Garp's big brother or even his father lool. Maybe it's because of Uncle Kong that Dragon left the navy?Maybe he was not on good terms with his uncle or his grandfather.
Monkey D. Kong.
Luffy have a Pirate Brother, Garp have a Marine Brother
That's why he really want them to enter in the Marines to be like him and his brother Kong
Maybe thanks to his Brother, Garp is not fired by the Gorosei.
It's all coming together.
What level of power do you place him in comparing with the legends of One Piece?
Will we see Kong afterwards? (Oda wouldn't have introduced / created his character and important status, he could have just been content with Fleet Admiral and not create an even higher status. Otherwise)
What will be its importance in story?