Parent is making an accounting point. Unless you have perfectly balanced trade, your balance sheet will continuously expand in either your currency, the foreign currency, or both. The Saudi government either isn’t inclined or can’t find counterparties to run enough of a trade deficit to provide the rest of the world enough riyals to satisfy their demand for Saudi oil. The USA on the other hand is willing to run such a trade deficit, it's pretty much the only game in town.

Meanwhile, the USA is able to find counterparties for its massive trade deficit because the USA itself has a huge and incredibly diversified economy and demands dollars in payment. You can buy oil from the USA in dollars for the same price you can get it from the Saudis. You can also buy all kinds of other things you can't buy from the Saudis. The dollar has massively superior optionality compared to the riyal (and most or even all other currencies) and that increases demand for it.