It's your FX-converted revenue, meaning, whatever currency you use converted to USD. The license doesn't bother to state this because they assume basic common sense on the part of the licensee.
If that's not enough, they have the backing of several decades of industry practice[1] and several centuries of law.
[1] For example, take a look at the Steam and Epic creator agreements, which also use USD for financial thresholds even though their stores operate in dozens of countries and accept dozens of currencies.
