Headcanon tbh, Sakazuki is a stronger leader than Sengoku, and they also recruited more powerful fighters via the draft.
If not and he's like 2 Billion and Kuzan was seen as Fleet Admiral candidate unlike Brocolli, I would definitely not see that.
Because real Black Blade holder is seen as 3.6 Billion belly for his combat strength alone.
A former Admiral qualified to be Fleet Admiral with inner navy secrets joining Enemy is still not seen as that.