So when the US bought gold other countries dumped their USD?
Last time the US bought gold as a reserve asset, the whole world was still under the gold standard. Currently, the whole world is under the fiat standard. That's the difference.
when they bought gold and the world went from a gold standard to a USD standard, because it was backed by gold...
Same could go for when the US buys BTC...
But anyways, every country has already the possibility to buy a harder asset than the USD, and still they all hold USD. I don't think that would change because of some BTC reserve