Yeah, but I don't think Airbnb has sophisticated enough infrastructure to figure out if every person leaving a review has an online presence before deleting their review.

It's possible there's something more mundane, like not having a browser cookie when the review was left or something like that.

Indeed. It's easy enough to funnel the money back to somebody you know, paying the 3% commission to Airbnb. There are probably tax ramifications too but as a business cost for a glowing review with no effort, worth it.