Yes, he's serious. On an Island where I lived (which was connected with a bridge to the mainland) a couple of teenagers in their car got lost dressed for a party in -20 celsius weather. They stumbled onto someone's lot, then got out of the car and walked around searching for help. (200 sq km island, lots of unmarked roads and nasty woodlands). By the time they were found one of them had frostbite and lost a couple of toes. They sued the owner of the land and the municipality because no 'no trespassing' signs were posted and won.

Pretty sad.

Not island related, but there has been at least one case of this that I know about in the UK: someone broke into a warehouse and was injured when a stack of stuff fell on him. I'm not sure if the claim would have held water in court but the company didn't want to take the risk and settled with him for an undisclosed sum. IIRC they were also fined for the health and safety violation that was the mis-stacked stock (as the same accident could have happened to an employee instead and an inspection found other such problems).

Warning signs disavowing the owner of a site of responsibility for other people's actions (in a public place or while trespassing) often have little to do with the actual rule of law though (except where the signs are mandated by regulation) - more often than not they are simply about reducing the chance of someone trying it on. You might win easily in court if it goes that far, but if you can save the time and other hassle of needing to go that far by putting people off trying it is likely to be cost effective to do so.

It's the same in France. If you have a swimming pool in your garden for instance, and you don't put a legal size fence around it, and someone gets hurt because he slipped or drown in the swimming pool is your responsibility.

It's the same for wifi. If you don't protect your network with a password, you are legally responsible for what happen on it even if it's someone in the street squatting your wifi.

You have to understand the underlying principles - you don't sue poor people, the ones who don't have anything, you sue the rich, the ones who have (and if their property is substantial, say - an island, the better). That is what makes sense and that is what became legal, justice be damned!

It's specifically called out in the wiki page linked to earlier, under the "United States" section.