That is one of those legalese complications. There are some special details for broker/dealers that I didn't pay a lot of attention to. The first 13g must be amended each time a 1% change occurs, from my understanding, then at 10% they become an insider, which the form 3 and 4 become involved. They can possibly file once a year unless they deal more than a certain percentage difference of their reported position. They can theoretically assign shares to long term and short term positions, and lower their base cost of lots.
I would assume they don't close the day far off of their recorded position, and trade as if they held no shares for the most part. They seem to lower the price when they need shares, and raise it when they are holding them. They will get out of the way if there is a large volume move in either direction. MM's know how to do this. They understand momo at least as well as we do.