It's not use to think too much about it, it's one of those chapters where Oda makes everyone act stupid because it was needed for plot
The problem is by doing so... he makes LOTS of characters who are supposed to be HYPED before this war... look incompetent before heading to this war
Basically it's wrong...
Becuase it doesn't matter if the alliance is going to WIN or get defeated in this upcoming First round of battle... they need to look competent before entring Onigashima
So... again I'm not against Kanjuro escaping with Momo... but Oda could have made sense out of it
Another example is for a bettr way for Kanjuro to escape with Momo... would have been him "NOT saying" that is mission now is to kidnap Momo... and deliver him to Orochi
Rather.. Kanjuro after revealing he's the spy... he takes Momo... and WHEN the alliance tries to get after him... he THREATENS to cut Momo's throat for example... which leaves them a little bit hesitent about whether he is doing that on his own whim to protect himself and escape... or whatever the case may be...
the point is threatening to kill Momo as a way for Kanjuro to GET OUT of that place... is a better way to make anyone makes ANY MOVE
Even if it was a "fast switch" shambles by Law's fruit as example... or Robin growing hands on Kanjruo and snapping him...
They would STILL THINK TWICE before making a counter move if his sword is on Momo's throat... as his means to escape this place safely... and kidnapping Momo basically will not be done as a MISSION... rather it will be something he accomplished by his OWN after his reveal as a spy
That would make more sense to me... also wouldn't make the alliance look bad like in this chapter