Yes, they could trade them to each other and count.
But how then would there be the disparity of 275K buys and 600 some sells.
If they are trading between themselves, it would be a buy/sell and both sides would add to the volume.
But it also could be true that 275K were sold by the market makers. 600K were bought by the market makers. which would signify net buying on the market makers part of 325K shares.