As someone who’s also worked in this area, I disagree with this take.

There is typically clear objective function of a recommendation system.

What Twitter is optimizing for is what’s of interest here. And some of the hidden business rules. It’s likely these are specified in the code in an obvious way.

How exactly they achieve that is the part that is complex and relatively indecipherable.

It’s possible that it’s designed in such a way the optimization objectives are also unclear, but that would indicate a bad design and be to the detriment of the company and users.