It's not like that cover could have waited a little until after Aokiji's role was officially revealed. Oda had to draw the Seconds back then and there and snucked in some nobody jailer to cover up for Aokiji (pretty much spoiling the picture now).
Anyway, if Kuzan is the real second strongest then the manga will let us know soon enough.
There are only two options:
- Kuzan will leave the group before they fight the strawhats.
- Shiryu is or will become stronger than Kuzan by the time the fight begins.
If it's option number 1, I guess Oda has spoiled us Kuzan "is not a real member and will leave". Which also sucks.
I think it will be the number one. Aokiji kind of said it to Smoker. Also, we don't know what his objective is.
Furthermore, members that have their agenda and then go away kind of mirror the Rocks pirates, which BB is clearly trying to imitate.
Also, but this is more complex reasoning, being the "second" doesn't always mean being the strongest, more so in that type of pirate crew-alliance. In CG for example, Buggy is seen as the leader, but probably is Crocodile that is managing everything, and for that seen as the second in command. Obviously Mihawk is way stronger, but he is not interested in this kind of role and work.