I don't know how much involvement I'd want Kizaru to have in this flashback. He could easily just be a cameo here, but what if he later becomes involved in Tiger's death? Why are his only major appearances outside of the war related to slavery and the Tenryuubito? Is he going to be a vital character in this storyline (that is, the slavery in One Piece not just the Fishman ISland arc), and later protector of those Tenryuubito again? Cause if so, that might bump Kizaru to one of the biggest bastards amongst the three Admirals.
All admirals are working for the celestrial dragons..those guy can summone one of the three all the time.
I think it`s just natural that some important marines were dealing with the Sun-pirates at that time, and why not taking Kizaru, who is well-known for us readers.
It would be a bit more weird, when a new introduced marine would be responsible for such a threat like those fishman-pirates.
But i guess that Kizaru will somehow get his admiral title because of the later actions in that flashback.