they're definitely using an OEM - it's just not possible for a 1 man business to design, manufacture, and develop firmware for a truly custom one-off ESC. With that said, one thing many OEMs are great at is allowing the customer to spec out a product however they want, so even if the ESC is coming from ZTW it's still more likely than not that you can't get the exact same ESC anywhere other than PPRC.

Besides, there can't be any more than 5 ESC OEMs in the world (honestly I'd be surprised if t was more than 3), but there sure is a heck of a lot more ESC brands being sold than that.