If they want to see a critical review of GiveWell's estimates, then they need to make a case that doing such a review is a good use of someone's time and resources, and also give some indication of what they consider to be an acceptable critical review. I mean, by all indications GiveWell is itself an independent, critical, reviewer of charities, so if they're not satisfied with it, why would they be satisfied with any hypothetical meta-reviewer?