Keep in mind that 129 tons of gold is worth just a bit more than $15b, so small market fluctuations on the scale of 10% isn't enough by itself.
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