I did my dissertation on this very topic (coming soon to my website... at some point). I do not know what tactics or systems Amazon has that deal with fake reviews, or if they even really bother beyond the easy wins like tracking IPs. My dissertation was on textual analysis, which is the hard part - the other signals like IP and behavioral related ones (like # of reviews posted in one day) are more fruitful.

The heuristic you seem to be using is a logical one, but requires more data analysis than Amazon might be willing to put effort into. Looking at these reviews, they are... well, so many on the same day is suspicious. My first theory is that some fake review writing company got tasked with flooding Amazon, and so reviews got farmed out to writers. Or someone has invented a GAN that writes good reviews, or at least a good first draft. I'd have to analyze the data to have more of an opinion. But yeah, verified purchaser means little.

The real question is, how economical it is for Amazon to really care about fake reviews? Buyer beware etc and there are more than enough scams running on Amazon / EBay / etc that I'm sure they're just treading water all the time. They have sued review writing outfits, so they care somewhat, but only after the problem got written up in enough newspapers... It is a hard job, trying to analyze all the data coming into their systems each day. I'm not sure any company has really implemented a lot of the research I read about in my literature survey.

ReviewMeta is another site I'd trust:

https://reviewmeta.com/blog/faq/