Read time counts as engagement and is measured in incremental steps, with the first 'engagement point' awarded at the 30 seconds stage. I don't know if you saw my previous article, but this explains it... https://medium.com/all-about-m/confusion-about-read-time-earnings-clarified-216b55c76ca9

And yes, it could still result in more shorter articles, and R4R being rewarded over quality.