this sounds more like a conspiracy theory and an scary one for that matter if it ever happens, which i hope not.
also i have to say i doubt if it ever happens because if it happens and it becomes clear that governments are hoarding bitcoin then bitcoin can no longer be decentralized and people will leave to another project. so that would be the death of that country's government.
I am not sure why a single entity hording BTC has to do with the decentralization of bitcoin.
Decentralization refers to who creates and values a currency, which would still be the Bitcoin community as a whole.
That is not to say that someone who owns a great deal of a currency cannot manipulate the market, but that happens with may currencies and commodities already today.
With a current market cap of about $10.5B, I don't think there is enough BTC for hording by govts to be worth while.
To the countries you mention $10.5B is not very much money.
For Comparison, The US currently owes China $1.3T.
Exactly. Unless the entire capitalization of the sphere greatly increases, any one single major government could 'buy out' the Bitcoin market. Hell, a few of the right major companies could do the job. This is one of the reasons why it wouldn't happen, as well as the fact it would be much cheaper and secure for said government to develop their own coin. To hell with all that anonymity and decentralization, from a national security standpoint.
I just don't think the amount of FIAT money we are talking about would be of interest to any entity that has the resources to truly impact the market.
Since BTC is not controlled by a central entity there is not enough predictability to make it more appealing to other options that exist today.
