My interpretation was that the author meant "even if we tried to review all changes between current HEAD and the last known-safe commit, it would be difficult if there were large commits (e.g. a giant refactor.)"
Then just fuck it, reset to last known good commit and work from there. If a contributor gives a shit about their PR, they will resubmit it again. If not, good riddance. If something has to be reimplemented again, it's usually way better the second time around anyway.