The same argument applies to gold i.e. governments and central banks buying gold is just like a hedge fund making bets while paying off gold retail holders with taxpayer money.

This is Status Quo Bias.

If you accept that Bitcoin, like gold, is a hedge against devaluing currency, then it makes sense for China or US to stock up on Bitcoin just like they stock up on gold.

Unlike gold, Bitcoin has a non-zero chance to supplant (or at least become a viable alternative to) US dollar as a global currency. In which case it's strategic for a country to own as much of such asset as possible.