When something is "realized" is a matter of accounting. It means to make the change, they sold the gold fo currrency, then bought it back. For many of us, realizing a gain is when taxes happen, though I'm not sure what it means for a nation state.
https://www.investopedia.com/terms/r/realizedprofit.asp
So they could sell it again and buy it again and realise another $15b?
So they could sell it again and buy it again and realise another $15b?