I actually hope that this is really UniJoin's business in China, because there are many people who have mixed needs for USDT.
I don't know how the USDT mixer works, but after Tether receives requests for a suspicious address, they can blacklisted addresses and then any coins sent to it can be burned or re-issue to the first party. USDT mixer would be useless, Tether could blacklist all deposit addresses and any coins sent to mixer might get burned so I don't know how there would be a mixer in a centralized blockchain