Well, they can buy down 21 million BTC and keep all of them to themselves, essentially 'killing' Bitcoin.
I would love that SO MUCH
Fact is that if they tried ( I mean at current price 21 million BTC is just 100billions, that's a budget tons of states could afford) they could probably buy... I don't know but 10 millions? Something like that? If they tried to hold it btc would instantly take +500% value!
To be honest that could be a smart move from a state... Getting a bunch of btc would be both a security for future and a rather good investment if they want to go back to fiat.