They probably wouldn't have been able to do this in the past, because a sell order was entered at every price. Furthermore, things aren't as easy as they used to be. With many software programs, the sale price will instantly rise as soon as there's an increase. Furthermore, there isn't just one Bitcoin exchange in the world.

To give an example from when Bitcoin's price was between $5,000 and $8,000, the number of Bitcoins in circulation was approximately 18 million. While mathematically it might seem like you could buy all the Bitcoins, Bitcoin's market value is the most recent selling price multiplied by the circulating supply. In other words, with the increase after buying a few thousand Bitcoins, Bitcoin would no longer be $8,000 but likely reach $15,000-$16,000. Furthermore, not all Bitcoins in circulation are sold on exchanges. Many Bitcoins are HODLed and sitting in wallets. While theoretically possible, this is practically impossible.