My memory of the dialogue is too hazy to carry an actual argument here... Do transcripts already exist online?
If the heroes came to know that fuel cells had been removed, they'd probably know everything there was to know about the modifications, save perhaps for minor details such as hidden switches or transponders, or software surprises.
One wonders... Did Khan really leave death traps in those torpedoes? Or did McCoy and Marcus simply fumble the opening procedure so badly because they thought
the things would be booby-trapped? It wouldn't appear sensible for Khan to leave the torpedoes with the ability to blow up, especially when tampered with. Yet demonstrably they had enough oomph to harm the Vengeance
...Unless Spock and pals actually installed
proper explosives aboard the torps for the first time when removing the corpsicles.