The first sentence of the article describes it as a "multinational operation" and the title credits both FBI and "partners". That makes it pretty clear it wasn't just a US agency operating with the authority of a US warrant.

They didn't access any random computers outside of their jurisdiction, only Qakbot servers.

I must have missed it, but does the article mention that those servers were all located inside the US? (where I supposed FBI has jurisdiction for stuff like this).

> As part of the operation, the FBI gained lawful access to Qakbot’s infrastructure and identified over 700,000 infected computers worldwide—including more than 200,000 in the U.S.

"Lawful access" is doing a lot of heavy lifting, but at least they specified it.