I think the author is right.

There are many peripheral nice-to-haves you can get from a code review. Bugs, security, performance, correctness issues are all possible bonus findings you may get sometimes.

But there is only one _must_ in reviews: another person reading and understanding the code, possibly suggesting architectural improvements, or asking questions that should be answered by rephrasing the code for clarity, or by adding code comments. In other words: maintainability. That's the one thing that's not a bonus point, and is a constant for all code reviews.