I worked on the things you mentioned at Instagram, doing the tuning this-and-that for about a year on a few different surfaces including the home feed.
You are right that for the most part, people look at metrics and make decisions about what to ship, and only the best engineers and data scientists spend time thinking about the actual product.
However, you're wrong about what metrics are important. Since at least early 2020, and to some extent since 2016, there is a hard and enforced constraint on so-called "wellbeing" and "integrity" metrics. Facebook actively measures the sort of things reported in the WSJ piece (self-reported wellbeing) as well as many others like "bullying" (as measured by human reviewers), "known misinformation", "hate speech", etc.
When engineers make changes to feed, they are generally not allowed to regress these non-engagement metrics. The focus of many shipping conversations is how to address even unmeasured potential risks to these metrics. A huge number of experiments are run specifically targeted at improving these metrics.