I’m sure they didn’t sell it in one transaction. Funds either have their own specialist execution traders or outsource that to another firm (Citadel for instance offers execution services).
Execution traders (and algos) entire job is to get in/out of large positions with the minimum amount of slippage. They due this by a lot of different techniques but at the minimum they will break up the position into lots of small orders across a variety of exchanges, dark pools, partners and other venues. Good execution traders will use their volume to create momentum swings in the symbols they are working on to get better prices than they would with small volume (running stops, filling in thin layers, etc.). It’s a constant cat and mouse game between them and the market makers.
But you don’t need to be a good execution trader to unload into GME right now, its effectively free for a manager like MUST. There were price levels getting cleared of hundreds of thousands of shares at a time yesterday. All this retail volume and the blown up price means that someone wanting to clear a ton of shares has ready buyers who are not price sensitive. And with so much ROI on their books they don’t need to sweat small slippage like they might in a less overbought symbol.