One of the problems is that lithium ion batteries can't be discharged fully without destroying them. Effectively, the batteries are a very large energy source that can never be fully turned off. Another problem is that (if I understand correctly) lithium ion battery fires tend to produce their own oxygen. Another problem is that a manufacturing defect in one cell could cause the whole pack to catch on fire. According to the article, a fourth problem is that roll-on/roll-off car carriers just store a bunch of cars in one big space, and there aren't barriers to prevent one car from starting all its neighbors on fire.
Putting out an EV battery fire basically entails dumping enormous amounts of water on it to keep the temperature under control until it's cool enough that it doesn't auto-ignite as soon as you stop. That can take hours.
I tend to agree that the risk of EV fires is overblown. ICE cars catch on fire all the time and we don't usually hear about it because it's not all that rare or unusual. It just doesn't make the news. EV fires are a novelty so we hear about them. (Not that any amount of vehicle fires is okay.)
I do think though that if you're moving thousands of cars together on a ship, then that's maybe creating a different kind of risk profile.
I look forward to wider adoption of LFP batteries. They aren't immune to fire; I think the electrolyte is still flammable. They just aren't nearly as energetic when they burn, and they're harder to ignite in the first place.