Large sell order like that often filled by selling a certain quantity daily for a certain number of days, in this case here maybe they sell 1/2 million shares for 50 trading days. Each days selling does not get printed until the entire order is filled then they print the whole thing (like it all traded in one day even though it did not).
So while filling the large sell order that creates daily selling pressure that ends only when the order is filled, which kinda explains why for a while we traded down no matter what gold price was doing.
It could have been a negotiated trade all at once also as you mentioned, they would have to find a buyer though without tipping people off they were selling their position, so not easy. .