Thank you. I do need to check their top page.Check out two things:
Their top pages by traffic on Ahrefs. Maybe they are ranking for some low comp very high volume keywords. If a few keywords pull in all the traffic then the stats are skewed. If they have a lot of keywords pulling in traffic that's a good indication that they can rank KWs
The other thing: do they have great social media presence? Are people engaging with their posts. You said they want a product for review. Let's say you already have reviews by high DR companies who will rank in the first page. Adding another site to second page won't do much. BUT if they can regularly pull 1-2K people from social media it might be worth it.
If you have no reviews yet from authority sources go for it. Since they will be the only review they will rank and whenever people search for your product + review they will click this site.
Hope this makes sense.