First thought: Maybe they bought the gold first? Or the gold price was at a temporary high when they sold it?

Second thought: The numbers don't seem to check out: 129t are 4,147,456.307 troy ounces (1 troy ounce = 31.1034768 g). The total gains of 15e9 USD would thus correspond to gains of $3,616.68 per troy ounce, which seems excessively high, given that today's gold price is at ~$4,712. Even if they sold everything at the current all-time high of $5,589.38 on January 28 (and that's a big if), they would have had to buy for not more than $1,972.70, a price we last had in fall 2023.

They must have had an exceptional crystal ball!