The volume of the futures is relatively low, but manipulation is the other way around if it happens (im not sure). They might have been buying real bitcoins in the run to 20k.

When the futures opened they could take a leveraged short position on the futures and dump the real BTC and manipulate the market down all the way to the future close.

This way they made profit on the real BTC by dumping it exactly on the top and make another hit with their futures (perfect short). Once down and futures expired they can start Buy cheap real BTC again while opening a long contract and keep playing this game.

I have no idea if this is happening, but this is the theory. So the manipulation isn't done with the futures, but with real BTC. The futures are the way to profit from the manipulation.