But there is no such thing as a “genuine” $30 bill.

If it's being passed off as money, then someone thought it was. I don't think the Secret Service cares if it's an invalid denomination or has Bozo the Clown on the front. Probably not a high priority for them given the overall lack of believability, but the attempt is what counts.

If you’ll allow yourself to go one step further in the pedantry, there is no such thing as genuine money either.