If you had a steady investment opportunity with 10% return (about in line with long-terms stock market returns), $9000 per year indefinitely is worth the same as $99000 now (in an idealized finance world. In the real world you can't invest $99000 and withdraw $9000 per year because withdrawals during downturns will take out too much. But it's a quick way to calculate equivalent values).

That's obviously an upper bound, because those domains won't make $9000/year forever. But valuing them at $10k if they make $9k/year is equally unsound. Not to mention the domain is worth more than its ad revenue. You could also end up selling it to a company that came up with the name and saw that the domain is available for purchase for some reasonable 4-5 figure amount (like in the example of this very article, where someone buys a domain for a five-figure amount)

Obviously there is a lot we don't know (is the $9k pure profit or are there substantial costs? How likely is the domain to sell?), but it sounds like the seller got the better end of the deal. He got more than $40k in value, in return the author got a deal he could afford