The only time that FIGs die immediately with a sunk AC is if the AC is doing the ATTACKING and the FIG(s) on it belong to another nation from your side. That is the only time that FIGs on an AC are considered “cargo”
If you are the one that attacks an AC with multinational FIGs on it, then it is just like any other battle against multinational forces, ALL forces defend against the attack.
So, the SUB vs. AC w/FIGs breaks down as:
1. SUB Attacking AC w/ FIGs: Both AC and FIGs defend. SUB can of course only score hits on the AC not on the FIGs. If the AC is sunk, the FIGs may continue to fight as long as the attack continues. After the battle, the FIGs may move ONE SPACE in an effort to land. If they cannot reach a legal landing zone in one move, they die.
2. AC w/ multinational FIGs attacking SUB: FIGs from a nation other than the owner of the AC are CARGO for the battle and cannot attack. If the AC is sunk, the cargo FIGs sink with it. If the FIGs are from the same nation as the owner of the AC, they attack and move normally for the combat.