The evidence that they didn't review it, is that a million line rewrite was merged 8 days after it began being written. It's simply not possible for a team that size to review that much code in that little time.
As far as testing - yes, they do have a test suite that it was checked against during the rewrite, but that still means that any behaviour that wasn't strictly tested for by that suite could have changed and it would still pass.
As far as testing - yes, they do have a test suite that it was checked against during the rewrite, but that still means that any behaviour that wasn't strictly tested for by that suite could have changed and it would still pass.