Why would a miner who is not bitmain use ASICBOOST on antpool? I don't think it produces more shares, so anypool would pay out the same as if it was not being used, and additional investment would have to be made on the part of the miner.
Because you need a stratum server that supports ASICBOOST for it to work. Currently only antpool does, but there is rumours slush may add support for it. It doesn't pay out more shares but reduces power consumption.
I'm not saying bitmain is mining on antpool. I have no idea if they are nor have I any idea if they use ASICBoost for their farms.
My understanding is that it reduces power consumption on a per block basis, not a per share basis. So any miner doing this would just find more blocks for antpool. A miner that was contributing 10% of antpool' hashrate that was using ASICBOOST would get a net gain of 2% of revenue and the remaining 18% would be split among the remaining 90% of miners on antpool.