u didnt get the point.marines object was never to stop 2 yonkou at once.wt they always aim for is to stop let them meet together.to do that they only need to intercept 1 yonkou on their way.remember when shanks and wb were to come in contact they only tried to stop shanks as shanks came to wbs territory.and it was mentioned that fleet was destroyed(which is obvious all of the admirals were present on marineford at that time).as for this scenario it big mom who claimed that she will come to wano.remember wt kaido said? "if u come here linlin i will kill u" further implies that it was big mom who was coming to wano to meet with kaido.so marines only need to stop big mom pirates on their way.so kizaru volunteered knowing he was the only one admiral who was left to stop big mom and her crew on their way probably.bt that itself speaks volume of admirals.they simply dnt fear yonkou..
continuing above, that scenario could hv ended in a worst way for marines which even they know it can happen.and that is both yonkou getting meet up with each other.bt here comes the best part akainu stopped kizaru not because of 2 yonkou getting meet up together(he never even mention them out of fear) he only stopped him coz he thought samurai 'the third party' can be strong which marines dont even know the strength of.now i dnt believe for a second that an admiral can take out 2 yonkou.i think both of them are pretty much at same level of strength.so yonkou beating 2 admirals or vice versa is absurd at my point of view.so lets say there were no samurai as the third party,then even though kizaru cant stop 2 yonkou he could hv intercepted them and could hv retreated saftely with minimum damage(can be way more considering wt happens bt definitely alive) if such a worst case to happen.bt with that third party i think the reason akainu refused kizaru's offer because if that worst case were to happen then pretty much akainu thought kizaru wont come back to marineford alive.
btw with what was shown kizaru only has kind of lazy personality.bt shogun was shown as more arrogant prick.so obviously if both of their statements were to take into count then the one with more arrogant nature will obviously be shogun's statement knowing wt kind of a guy he is as shown.when kizaru made that statement he was obviously very serious.its not like kizaru is the type of guy who wouldnt acknowledge opponents strength.we saw him praising weevil and marco at marineford for their strengths.(remember this topic rose again due to the fact that Paaserby post wt shogun said as a confirmation to prove that yonkou>admiral.thats y i insist on shogun's statement and its flaws)
u may seems to forget that both of them made that call knowing the WG was busy with reverie.
You must be registered for see links
here u can read it.so no matter wt way u look at it.this scene works on the favour of admirals more than yonkou