If central banks sell their gold in the US and buy new gold abroad (to save themselves physically transporting the gold across the ocean[0]), I suspect that the demand this creates for gold abroad will ultimately cause US gold exports to rise, too.

[0]: See the link in https://news.ycombinator.com/item?id=50134172

Selling it in place neatly sidesteps suspicion about how secure the NY gold really is and if it's all really still there, too.