How do you explain the screenshot then if its a fake?
The same way phisher make fake websites to look like real bank sites or other sites that steal your credentials. I don't do it so I don't know, but I imagine that it can be done. I'm not sure if it's a coupon or not, but I can't select a coupon from that page on the Bitmain site. The only time I can select a coupon to use is during checkout but he doesn't say it's a coupon, just a loyalty discount and if this does exist who even knows if Bitmain allows you to transfer it to another user. I don't know if it's legit or not, I'm just saying proceed with caution and always use escrow.
To be honest i dont think Bitmain would do it in the first place, ever. They dont even offer such discount in 500-1000 MOQ orders. And if they did, i really doubt they could write that many english words without writing them in a 90% broken English.