This is just the latest in a long line of landings on the wrong runway, on a taxiway or even at the wrong airport.
The photo is a bit misleading as to get an idea what the pilots might have seen you need one that was taken on a 3deg approach a couple of miles out, not a god’s-eye view from one side.
The most effective “closed” sign I have seen was a big “X” made of bright red lights stood up vertically just in from the threshold. You could see it from miles away and it would get more and more obvious as you got closer. It was also mobile, so could be repositioned while work was ongoing.
I’m sure the crew of this aircraft will be asked a fair few questions to try and find out how they managed to land on an unopened runway but I’m also sure they didn’t set out to do that. If there had been obstructions on the runway, I don’t think they’d have landed either.
It wasn’t the most professional piece of flying in aviation history but I wouldn’t label them as idiots, certainly not yet.