Bullshit, it's obviously exactly what Fam was making an attack run on.
It may have been lucky that Millia actually hit what she was aiming for, but it's not like they were just shooting the enemy ship randomly and just happened to hit the self-destruct button.
Right, which already makes it a cliched plot device, which is made worse by the fact that Fam didn't even steal secret Death Star plans for this ship beforehand to know there was a weakness there.
Which means, it's probably not a secret that it's there, yet nobody bothers to shoot at it with trained marksmen instead of spoiled princesses.
Which begs the question, why the hell is the inferior vessel the flagship?