They purchased 129 tons of gold in Europe. Their asset position did not change: they converted cash to gold of the same value.

They then sold the 129 tons gold in the US vaults for $16 billion. That gold was originally purchased I'm guessing many decades ago for $1 billion. The have a book profit of $15 billion and still have 129 tons of gold.

They captured some of the appreciation in gold value as a realised profit on their books.

Their balance sheet did not change, just their income statement