Because your local agent can already see the full codebase; the code review only needs to see what's changing and evaluate the change.
I found diff reviewers so heavy in false positives, all my coworkers just ignored them.
The way we set it up was that the code review responded to two signals:
1. GH PR webhook
2. The exact same review agents running as an MCP tool that the local agent can invoke before pushing.
Practically, what this means is that it's OK to have a false positive because the local agent can make the check with the full context.
This would be the same as if the entire team used Codex and, for example, had a sub-agent configured to run code reviews using a smaller model. In this case, the benefit to this tool-based approach is that the exact same agent works for all harnesses across a team and also works in the PR itself.