That's true, those numbers don't work out for Google. But they have essentially unlimited resources to discover the exact threshold at which that person is just barely incentivized to keep their site active. $100K/year, reduced 80% to $20K/year? Still enough for them to keep their site up part-time? Etc.

The bulk of the traffic they're referring is essentially residual profitless goodwill left over from their "don't be evil" days.