So then, accumulation can never happen?

I think someone sits there with a buy order, limit price, and waits until the MMs finally fill, likely out of their inventory.

The past few weeks we have seen similar trading, large block trades among the 1K trades, that stick out like a sore thumb, but they go unnoticed, for the most part, as they do not affect the pps.

If it was a sell I would expect the MMs to fully take advantage of the situation. Go ahead, put in a market order to sell 400K and see what happens.