There's no dark pools in these markets (or at least, none that are liquid enough) so realistically speaking, selling a big lot like that would affect price regardless as other people can see all the orders go through. Add in the fact that liquidity is so fragmented and it's fairly difficult to sell such a huge lot without impacting the market regardless. Also, I suspect a lot of that volume was the same Bitcoin(s) being traded MANY times so it doesn't really represent the true supply & demand situation in the market - a sudden influx of supply would affect price even in smaller lots. Still, smaller lots probably would have made more sense so who knows for certain.