Did he actually say he was a US operative? Or was it his lawyers arguing that IF he is as those suing him allege, then the suit should be thrown out since he would have immunity?
The rumor on the street is that he was probably working for the UK. This is one of the reasons Trump was not thrilled with the Brit government before Boris.
If Mifsud were anything but a US asset that would be the biggest spy scandal in years.