To what degree do we expect intellectual peerage from someone just glancing into this problem because of a PR? I would expect that to be the proper intellectual peer of someone studying the problem, it's quite reasonable to basically double your efforts.
Generally if the reviewer is not familiar with the content asynchronous line by line reviews are of limited value.
Having somebody else devote enough time to being up to speed enough to do code review on an area is also an investment in resilience so the team isn't suddenly in huge difficulty if the lone expert in that area leaves. It's still a problem, but at least you have one other person who's been looking at the code and talking about it with the now-departed expert, instead of nobody.