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

Do they count as export if the owner doesn't change?

I don't know. Does it count as export if it's not moved an inch?