It is so tiring to reply to the hordes of ignorant trolls.
I wrote upthread that one could buy and sell the coins on an exchange. They would then hold the historic private keys to attack with. This would only cost them the average spread between buy and sell prices, so they don't actually have to buy 50%.
Even monsterer doesn't claim that collecting historic priv keys is a viable attack vector. It was explained why it isn't. He claims that it's easy to collect enough priv keys for this attack in a short timeframe.
The attacker buys all keys at once, or very close together as stated in the description.