I never understood this idea that Bitmain was BUYING BCH. I believe they have only been accepting BCH as PAYMENT for their machines for a while now. So, if you are selling a crap ton of machines and receiving BCH for it, then the amount of BCH you OWN is going to go up significantly. YES??
Well, for quite a few months now, they are back to accepting other forms of payments. However, a good chunk of the BCH stash that they had back in March can probably be attributed to mining and proceeds from sales. Also, they likely sold most of their BTC to keep their operations going, which have included several failed attempts to get a new SHA256 chip developed. I doubt they did much buying of BCH off of the markets, like some people accuse them of. The reason that I doubt it is because they can accumulate for much cheaper by selling their equipment for BCH and mining.
Bitmain has never sold a single BCH they have mined. This has artificially supported the price of BCH which has close to zero organic demand. They have sold their BTC to pay the electricity bills.