- if Fujitora were sent to Boa, why wouldn't say It clearly and only show Koby? For the suspense only?
- we know that even if Boa were the strongest she could be, she wouldn't stand a chance against an Admiral, much less against Fujitora who is a Natural counter to her. If Fuji beats her, nothing really changes: it's only given.
On the other hand, if Koby does it not only it's big news, but Koby gains an entirely new role;
- imho, it was long implied that, after the downfall of Smoker, Koby Is the actual new generation Marine: not only Garp disciple, but nicknamed "Hero" like him, he has promised to fight with Luffy in the NW, it's a SWORD member, etc...
- last but not least, Oda likes to "surprise" the readers: as I said before, an Admiral beating Boa it's predictable, "normal", the unpredictable would be Boa's victory or at least her escape; on the contrary, Koby's victory would be much more of a plot twist.