The whole point of TFA is that it doesn't matter what code the developers write; the browser needs to block on handlers regardless due to how the events are specified. The described new feature fixes this.
But wouldn't a developer who is surprised by performance penalties caused by empty scroll event handlers also not use passive listeners?
The point of passive listeners would be to make it easier for developers to not accidentally write shitty code and boost performance on well written code that doesn't have to be blocking the scroll.
Chrome gives nice warnings when passive listeners are not used[1]. This is demonstrably[2] causing people who have not used passive listeners / may have written poor code in the past in the past to take note and use them. It works.
The point of passive listeners would be to make it easier for developers to not accidentally write shitty code and boost performance on well written code that doesn't have to be blocking the scroll.