Remember that if they bought, they'd buy over-the-counter direct from a miner, instead of using exchanges. So they'd likely not move the price much.
We mine 1800
BTC a day, 10 billion$ mean 2 million
BTC or the mining reward for more than 3 years for all miners.
No over the counter deal would accommodate this, we're talking about 1/9 of the total circulating supply and probably 1/7 or 1/6 of the whole coins that people still have access.
Before the price rise, we would see warning signals from the banks involved in moving such sums.
What evidence do you have that it was in fact them? Some obscure Russian economist made a claim completely out of the blue (read, straight out of his ass) and half the forum went wild with Russia allegedly buying a few coins.
The rumors came when we didn't have any real hope of any price rise for the short term, so people clung to anything and hoped for the best making headlines out of anything insignificant, Russia, Samsung, Amazon,BMW, etc etc and now that we have a rise thy think they nailed at least one of it.
The problem is that even if they can't prove their theory, we can't dismantle it either, so at this point is turning into a futile maybe, what if, it is, no it isn't discussion.