That is the way whack is using it. whack is correct that if there is a negative effect on the average user, a test will show that negative effect. That's what "average" means.

To perceive an effect in new users without getting the same effect in existing users, you'd need to show different content to those two groups.

Hmm, I think the authors point is more towards attention addiction, rather than specific average types of people. It’s more a matter of setting a low bar to encourage more people to be distracted by your app when they really shouldn’t be using it. Basically increasing the number of apps that people check in on, especially when those users are in their marginal time (before bed, while cooking, etc.).