The spolier say "send an admiral or the goroseis" it never say an admiral= all 5 gorosei.
In the conversation with Imu the elder were simply telling imu to not trouble himself and that they will send an admiral and if the worst come the Gorosei will themself go to elbaf.
That is all there is to it.
How you all turned thus into 1 Admiral= 5 gorosei that is a mastery for me lol.
As if a young Xebec didnt kill an admiral no diff in a flashback