I agree on both counts. In fact, the examples of constructive feedback the article provides seem fairly worthless. If your advice is that the candidate should try doing some problems on leetcode in order to be successful in your interview process, then that's something you'd ideally mention prior to the interview, as a suggestion on how they should prepare.

I think FAANG do a really good job of preparing you for exactly what kinds of questions to expect as well as resources to study for them. That also generally reduces the need for feedback afterwards.

Generally no amount of new information will change someone's mind about a date but in hiring that may not be true.

This is the answer.

What you can get out of an interview is a lot of information about the competence and culture of the company. Did the interview feel like a real attempt by good people to hire someone like you into a company you'd like to work for? If not, put them on your ban list and never look back.