I don't think you're understanding how global taxation works. Billionaires would be taxed on their global income/wealth and the proceeds would automatically be divvied up to each country based on a pre-determined formula of which countries deserves which percentage. Obviously America and the other big nations will try to screw the smaller ones in the negotiation, but it's still better for society to receive its fair share of the wealth, especially because it's the broader public that creates it all anyway (via workers, consumers, infrastructure, education, etc)

(Personally, I'd rather see a fairer pre-distribution of wealth so billionaires never exist in the first place and there's no need for a wealth tax, but that's another story.)