It means that if you offered him betting odds that it would happen, of less than 1:2, he would take it. And he would bet against it at odds higher than 1:2. Of course betting on the far future is impractical, because money might not be worth anything if it does happen, but in principle that's how these numbers can be interpreted.

The danger is that they then announce these numbers as if they have any real-world basis, and that they are taken as making a claim about objective reality.

There's no evidence I know that pulling a number out of your backside and running it through Bayes' theorem produces better results than just pulling the number out directly and running with it. Indeed, the real-world track record of LessWrong/MIRI (to bring this back to topical relevance) strongly suggests it's less effective.