If you're reviewing a block of code, you need to review the whole final version (not just the diff) or you risk missing context and interactions. If you're doing that, it's simpler to just review the whole thing and not filter out some parts just because you later notice that it predates the current patch. Although I would tend to hope that he doesn't treat pre-existing issues as blockers, I can certainly see why a person would take that approach if the goal is code quality.
