I tend to agree,

If you have invested significantly in the planning phase and there is momentum in the architecture and conventions that already exist in the project, the implementation phase might not need as much oversight as is suggested here.

> You can discover that your initial idea was dumb and a better one exists

The planning and architecture phase is usually where I make these types of discovery at a high level.

> Your agent might go “off the rails” and start doing something you don’t want it to do

Candidly these orthogonal, inadvertent edits aren't as bad as they once were and for impactful changes there should be at least some test coverage, even if that test coverage is just "freezing" what was implemented.

As you mentioned the final review discussion is a good chance to verify beyond what review or adversarial review agents find.