They monetized an existing accounting/revaluation gain by selling older, non-standard bars and replacing them with compliant bars, while keeping the overall gold quantity unchanged. That is not the same as "we moved gold home and earned $15B on the move."
In simple terms:
- You buy x of gold at $10
- You sell it much later for $100
- You made a profit of $90, and you hold $100 of cash
- You rebuy x of gold for $100, back to the same gold exposure, but on the books, you have $90 of profit