Turns out the number is entirely based on a quote in 2006 from Fortune editor Jon Fortt:
“The online giant figures that Google News funnels readers over to the main Google search engine, where they do searches that do produce ads. And that’s a nice business. Think of Google News as a $100 million search referral machine” [1]
He claims he got the number from Marissa Meyer at a tech conference lunch session, but without quoting her directly, and there's zero detail about whether it is supposed to represent revenue or a long-term valuation of the business.
Then the "study" says $100M was 0.7% of Google's revenue in 2006, and assumes it's still proportionally the same, which makes it $4.7B. [2]
Which is completely ridiculous. Google is a vastly larger and more diversified company now, but more importantly hardly needs News to drive users to Search. (Back in 2006 when Yahoo was still a major competitor maybe it did drive meaningful traffic to Search that wouldn't have occurred otherwise, but that's very hard to argue today.)
[1] http://fortune.com/2008/07/22/whats-google-news-worth-100-mi...
[2] pg. 23 of http://www.newsmediaalliance.org/wp-content/uploads/2019/06/...