Btw - I never found a good explanation why all USD transfers need to be routed trough usd banks. Google is quite silent on the subject - is it common knowledge or too obvious?
However when you're talking about billions, it's difficult to keep them in anything but US treasuries.
all usd swift wires get routed through 2 of the major banks - think it was JP Morgan Chase & Wells Fargo. this allows them to check all of these transfers for aml/cft and other sanctions blacklists.
heard this from a guy who works closely with Tether so i'm sure he'd know well