It’s not a scam, I get a few of those and take them up on it. You can get some good stuff for free. It’s a bit of a faff, you need to send them a screenshot of your review once it’s visible on Amazon, but they’ll refund to your PayPal.
Generally you have to be very specific with the search terms, they will tell you what to type, and to screenshot the search. The review doesn’t have to be favourable, or detailed and long, just anything.
Best thing I’ve got, not from Sofirn, is a 4G enabled trail cam. What a game changer not having to go and check the thing manually. I used it in the loft to keep an eye on a rat trap, much better than having to climb up every day.