Being the reserve currency, every country wants dollars. Every other country needs dollars to do trade, even among each other.

With a tax cut, US consumers have more money which they spend on imported goods. This supplies dollars to the rest of the world, increasing the value of 196 currencies marginally. You can think of USD being in demand from 8 billion people.

If US dollar wasn't a reserve currency, a tax cut would have increased supply of dollars with consumers but demand would only be from the 330 million people in America. High supply of money, no change in supply of goods would then cause inflation.