That's what Ad Age says, citing anonymous sources.
Here's how CNET describes how a data exchange works:
The services allow Web site owners to sell data on their users to potential advertisers. So, rather than buying a single banner ad at the top of the page for all to see, data exchanges allow advertisers to target specific portions of the site's audience, such as stay-at-home parents living in Montana or people shopping for a new smartphone.
So Google would be the intermediary. Content providers would have the details on their users and they would sell that to the ad companies who could tailor ads. This is one of those good news bad news things. If you're going to see ads, why not have them be about things you actually enjoy? And also, why shouldn't you see ads in exchange for the content you're viewing, which has value? On the other hand: creepy! That website is selling my details to ad companies?