That email thread you linked is between the Ads and Chrome (not Search) teams, is about the number of search queries (not the results of search queries), and “ranking tweaks” there refers to the ranking that Chrome uses to show the suggestions in the omnibox (address bar). (To get a sense of these “ranking tweaks”, try this experiment in a (new?) Chrome profile with default settings: type "flowers" in the Chrome address bar and don't hit Enter, and look at the suggestions: what mix of search suggestions, entities, and bookmarks/history do you see? Try again with other commercial queries like “insurance” and “mortgage”, and also some less commercial queries like, I don't know, “Minnesota” or “economics”.)
(And FWIW, I think that whole email thread actually shows Google in a “good” light relative to the popular impression here on HN as a company whose every action is some Machiavellian scheme to increase ads revenue: it shows that Chrome actually launched something to production before its negative impact on revenue became a concern, that Ads leads had to work hard to persuade them to either roll back or find some other way to undo the decrease in search query volume, that starting to include search query volume as a launch criterion would be a “cultural shift” for Chrome, etc: that Ads having an influence on Chrome is a rare occurrence.)