I never said it's a stand-in for actual user studies. I said it's critical. You need both. User studies will never give you the broad, hard data you get from engagement tracking. They also won't give you as fast feedback. People have all sorts of biases in how they present themselves in user studies, and you should get that data and try to sift through it, but it's not a substitute for actually seeing how people behave.

Also, engagement tracking is actually really hard to do well, so it's definitely not lazy. But it definitely is shallow.