He must be talking about its user reviews in Google Play, it's mostly 1-star with 1.3 star average.
I can see 3.6, so it also seems to be device-specific
It is. Google Play reviews are device specific.
As to Bitmart, if you take a look at its reviews from a neutral standpoint--meaning,
no devices--what you'll see is 1.3 stars average out of more than 50,000 reviews. It's worth noting that out of the 20 reviews at the top, Bitmart got 18 one-star reviews. Those are phone reviews.
They fare better in Tablet reviews. They got 3.6 on average out of just hundreds of reviews, and 6 non-single star reviews out of the top 20.