Does humanity really want an authority powerful enough to claim dominion over the entire planet?
Bloomberg has a better write up: https://www.bloomberg.com/news/articles/2021-04-08/plans-for...
To answer your question, this wouldn't create a new organization to collect a global tax. Instead it would give the IRS the authority to tax American companies (say, Google) an additional amount on their overseas earnings. So, if they paid 13% in Irish tax, but the American tax rate is %21, the IRS could charge them the gap (an additional 8%). This would be true in all countries that sign the agreement.
Obviously tax havens would still exist, but they'd be harder for big multinationals (like American tech giants) to abuse.