Suppose a very rich Asian Businessman left Hong Kong 10 years ago because he had the foresight to see what was going to happen there today with the Communist take over.
Suppose he made all of his wealth from HK and then in 2009 MOVED a significant portion of his wealth to this particular Swiss Bank.
He then immigrated to the USA LEGALLY and moved the rest of his wealth to the USA and in 2020, became a US Citizen.
Does the IRS have claim to all the money he has in that Swiss Bank account seeing that the money was NEVER MADE in the USA?
No. The IRS taxes income, not previously and legally acquired capital.