They say not securing the cars properly can cause impacts that cause fire. But these cars are impact tested for passenger safety. The only vulnerability for the battery pack will be a bottom impact which should not occur on Roll on-Roll Off Carrier.
Ships do need to adapt their firefighting process for Lithium-Ion batteries. But I'm not convinced about impacts causing fire.
There's a couple videos on YouTube of vehicles on a carrier ship where the vehicles are bash around a fiar bit beyond what might happen in a passenger safety impact test.
My parents had their car parked right behind a caravan with a bike rack on the back - they didn't notice anything when driving off the ship, and so it was kinda too late to do anything about it by the time they noticed the impact damage to their car's hood from the bike rack bouncing up and down.
Doing that for thousands of cars, at boarding and then at unboarding, looks like a huge logistics chalenge (even just thinking about changing every ship’s floor to allow for strapping) that has a pretty high cost that might exceed losing part of the cargo once in a blue moon.
Shipping cars in containers sized structures could be a more realistic approach perhaps.
Initial manufacturing defects are likely a decent cause. And you only need one in ten or hundred thousand to still have issues if such can affect other vehicles tightly packed around.
One way in which battery packs catch fire: initial weld looks good and passes inspection but turns out to be faulty after all. It disconnects, then the pack i s charged (this should not happen to cars intended for overseas transport), vibration causes the bad connection to temporarily make contact, then the current welds the contact more solidly and large amounts of current start to flow from the charged cells into the one uncharged cell. Depending on pack geometry this can be 20:1 without any kind of protection mechanism in between. The one cell then catches fire, which in turn can set off the cells around it.
This is why per-cell fusing is a thing in newer designs and why weld quality is super important during manufacture.
I'm not sure what the inside of a car carrier looks like, but I think the idea isn't just that a little jiggle that causes the suspension to flex slightly is the concern but rather a car coming loose and falling on another car, or something similarly serious and unplanned that isn't supposed to happen if everyone is doing their job correctly. If it does happen you wouldn't want it to be a risk to the whole ship.
There's also the possibility that a battery just catches on fire all by itself because of a manufacturing defect. It can happen.
Ships do need to adapt their firefighting process for Lithium-Ion batteries. But I'm not convinced about impacts causing fire.