If such a trade is "done privately", how does the knowledge of it get into the marketplace... and for what purpose?
This smells like BS to me, notwithstanding Zerohedge being known for just making shit up in general from time to time. They probably just looked at the volume for several minutes around the quick drop and ginned up a cool sounding story. If it's all mysterious and no one can prove it WASN'T a single order from some "big fish", then they skate by with it. I can imagine there were significant orders sitting around the 200DMA just waiting to fire off...and they did.
Should actually be relatively easy to show it wasn't one big order (if thats the case). I have micro-second timestamped quote/trade/book data.... I'll dig into it once I get some time, and post back here. Most likely it was a big sell order that triggered an avalanche of stops.
I already posted a chart showing two thirds of move happened ... before the so-called big order could of hit.