That's a misleading statement. First of all at the beginning of the 20th century the British pound was clearly the global currency - where a lot of the international commerce happened. Then towards the 1930s and later on the Dollar took over that role. But that role carried much less weight back then - globalization was still in its infancy and a "global reserve currency" as we understand it today simply didn't exist back then. Right now some Dutch pensioners money is invested in American, Canadian, Australian, German, etc. companies. And the parts for most consumer goods are coming from all over the world. Pre Bretton Woods basically none of this existed. This global financial system we speak of today is unprecedented and only exists since the late 70s - where most countries abandoned their currency controls (apart from the US and CH which never really had them).
There is excellent research in "currency diversity", start with Prof B. Lietaer
So for instance yeah the US only has about 1PE (Peak Elon) of gold, but the entire global level of US reserves are only about 5PE!
If a single country's currency is the global reserve currency, all you're finding out is whose thumb you're under.