Although this is true; if the manager is thinking about and getting involved in architecture after the PR is written it does suggest something has gone wrong. If there are architectural considerations then it is good to discuss them with the coder before they start developing.
PR review is a great time to pick up subtle bugs, do last-line sanity checks or get used to someone's style but if they are a bad arena for combating most code issues. If they are picking up design problems there is probably a process flaw to be corrected.