No, they can't achieve control that way.
1. It's not possible to accumulate a significant proportion of the current supply without pushing the price up commensurately. And as is pointed out in the OP, the legitimacy is also pushed up. Incentive to sell back to fiat is further lessened.
2. Not all supply currently exists, so this could never be a one-off, it would have to be a permanent reserve purchasing policy, as otherwise their previous position gets eroded by the newly mined supply.
#1 is not an issue. The goal, as per the paper, is not to corner the BTC market or have absolute control over the price. It's to have a BTC reserve of sufficient size with which to counter private actors doing, for example, what Soros did to the pound, implicitly assuming those actors have limited supply of BTC as well.
It bears repeating: they won't be trying to buy the entire stock of BTC, or even a majority, or even a large fixed quantity. The BTC reserve would likely be measured in equivalent SDR value, or percentage versus other holdings. So if BTC price goes up as a result of their purchases, they simply buy less.
The BTC dues would have to be either:
1. bought in the open market.
2. confiscated by force from centralized storage pools e.g. Gox, using same excuse as in 1933.
3. not mentioned in the paper, but feasible if they get going soon and have a large ASIC budget: mined.
Either way, the obtained coins would be mostly hoarded, and so supply available to market would diminish. Hence any action based on this paper, is price bullish for BTC. And of course, once the IMF and other countries start paying dues in or keeping BTC in reserve, it would completely neuter the irritating drugs-porn-crime objections that hang over Bitcoin.
Yes, having a BTC reserve would enable potential manipulation of prices on short-term timescales. But as with gold, they cannot own the entire supply, so long-term fundamentals would manifest themselves regardless.
Again, the paper is not an attack on Bitcoin, or an attempt to change Bitcoin, or outlaw Bitcoin, quite the opposite. This paper, if implemented, is a giant positive and necessary milestone on the road to Bitcoin being taken really seriously as a peer to state-issued money.