"Furthermore, while Uber now allows riders to tip their drivers in-app, this did not become available until June 2017, which is outside the scope of our data. We do not believe that cash tips – which were possible before in-app tipping – had a material impact on driver earnings."

The study did not have any actual tip data for drivers. Tipping was always an option in Uber before June 2017, it just became mandatory for the app to ask a user after each ride. I really think the study is missing some crucial data surrounding tips.