The review system is rigged to nudge users into action rather than non-action. This means that when someone opens a review, if they want to review, there is a good chance that they will act on a low quality post, or a late post, and review it.
But since the community is not very large in terms of reviewers (there are only 114 Reviewer badges at the time of this post, and this comes with much repetition, as one can obtain the Reviewer badge up to 6 times), this means that the likelihood of the action being taken by someone who is not an expert is not negligible.
Sure, some stuff is clearly nonsense and should be acted on (e.g. spam, or other clearly not-answers). But there are times that I, for example, would flag something as low quality or not an answer, because I can parse the content of the post and I recognize it to be irrelevant, or sufficiently far from the actual topic. And there is nothing more frustrating to see this flag cleared by someone who has no actual experience in set theory clearing the post as "Looks OK".
Yes, on other websites, the ideal way would be to downvote or leave a comment that this answer is nonsensical in hope that reviewers would take that into heart, or other users would take that into heart. On MathOverflow, I sometimes find that to be unnecessary. Expressing open dissent can lead to unnecessary conflict, which can be easily avoided when the answer is deleted from the review queue or by the moderators. (And history had shown that at least on a few occasions people whose answers were deleted in such way are exactly people that I would not want to interact with.)
Since I doubt we can request that the review system is changed so you cannot review posts in tags where you have no active participation, can we at least interject (using the "extra script privilege") a banner reminding the reviewing users they should skip reviews outside their area of expertise in case the answer is not clear cut?
I am not saying that only experts should review questions or answers on their topic of expertise. That would be stupid and counterproductive altogether.
I am saying that people who are not experts should be aware of their non-expertise, and their inability to judge content which might seem correct to them, but is really just a bunch of nonsense, or something that is not worth preserving on the public face of the site.
By not reviewing and skipping, you make the flag reach the moderators, who then can either consult an actual expert—or more likely see that an expert raised the flag, or that it was raised out of pure spite and ignore it—and act accordingly.