This is not for AdSense on Google.com, this is for third-party sites with a search feature showing search ads to their own customers. A third-party site who wants to show ads in their own search results could partner with Google to include AdSense ads in their own results. The Google contract originally required exclusivity, and then was changed to require Google ads to be more prominent. In the context of the market…
Single digit percent of Google's overall ad inventory would be $1bn-$10bn per annum, at current scale. Google's ad business is big. Over 10 years...
One of the first "tricks" you learn on adowrds is to turn off (it's hard to find) "search partners," or at least split it off and quarter your bids. It's on by default and'll take up a surprising amount of your budget, using default settings.
I wouldn't be surprised if it ads up to a meaningful share of the "search ads" market.