Yes this is exactly what they do, and what happens with many Amazon items as well, and for Booking.com
It is not really a big cost because you only really need to pay the platform fee (you control both sides of the transaction do the only real cost is the middleman).
Reviews are super important and it is hard to get booking when you have 0 reviews so it is not surprising that some hosts would spend some dollars for 3-4 fake positive reviews to kickstart their property (many people will bounce off the ad without at least a couple positive reviews, so it changes your business radically)
Except you are describing the opposite of what is being done. Not positive reviews, but negative reviews. I can certainly see astroturfing your own properties with great reviews, but negative ones? That is a situation where you do not control both sides of the transaction, so a high price to pay to put a "fake" negative review on a property!
In the case I commented on they left a positive review. I'm sure many hosts will accept the Airbnb fees once to boost their rating. Especially if they're starting out, early reviews carry more weight.
Implying that a "bot" or "puppet" is paying to stay at AirBnB's just so they can write salty reviews?