Engagement is my favorite form of metrics pseudoscience. A classic example is when engagement actually goes up, not because the design change is better, but because it frustrates and confuses the user, causing them to click around more and remain on the site longer. Without a focus group, there's really no way to determine whether the users are actually "delighted".
EDIT: For some reason it didn't compute with me that you already referred to the same example. I've seen that exact scenario play out in real life, though.
I bet the reddit redesign used a similar faulty measurement of engagement.
"People spent more time scrolling the feed, people must enjoy it!"
No, the feed takes up more space, so now I can only fit 1 or 2 items on my screen at once, rather than 10, so I have to scroll more to see more content.
That would not surprise me in the least! In fact, that's exactly what happened at a company I used to work for (that shall remain nameless). At the behest of the design team, we implemented a complete redesign of our site which included changing the home page so that at most only two media items could be on-screen at a time, and the ads which used to be simple banners now were woven between the feed of items. I remember sitting in a meeting where we had A/B tested this new homepage, and witnessing some data analyst guy giving a presentation which included how "engagement in the B-group was increased by N-percent!!!" The directors of web content were awestruck by this despite no context or explanation as to why supposed "engagement" was higher with the new design. The test wasn't even carried out for a long duration of time. For all anyone knew, users were confused and spent more time clicking around because they were looking for something they were accustomed to in the original design. And no, it did not matter that I brought up my reasons for skepticism; anything that made a number increase made it into the final design. Then, we actually had focus groups, long after the point at which we should have been consulting them, and the feedback we received was overwhelmingly lukewarm or negative. Much of it vindicated my concerns the entire time; users didn't actually like scrolling. Then again, I guess if they're viewing more ads, then who cares what the user thinks?? Never have I felt more like I was living in a Dilbert comic than that time.
If that also resulted in little or no change in how often you (and everyone) opened reddit each day, then it is a "success" for them. They have your eyeballs for longer, so you likely see more ads.
If only they were trying to maximise enjoyment and not addictiveness. They don't care at all about enjoyment, just like Facebook doesn't care about genuine connection to family and friends, or twitter to useful and constructive discussion that leads to positive social change.
EDIT: For some reason it didn't compute with me that you already referred to the same example. I've seen that exact scenario play out in real life, though.