Got a source for that?
He renounced his U.S. citizenship and all benefits thereof derived when he pledged loyalty to Al-Qaeda, Al-Qaeda being at war with the U.S.
Sec. 1481. Loss of nationality by native-born or naturalized citizen; voluntary action; burden of proof; presumptions(2) taking an oath or making an affirmation or other formal declaration of allegiance to a foreign state or a political subdivision thereof, after having attained the age of eighteen years; or
(3) entering, or serving in, the armed forces of a foreign state if (A) such armed forces are engaged in hostilities against the United States, or (B) such persons serve as a commissioned or non-commissioned officer; or
It specifically says "foreign state or political subdivision." But the end of USC 8 Sec. 1481 really puts that it of contention as a binding statute.
(b) Whenever the loss of United States nationality is put in issue in any action or proceeding commenced on or after September 26, 1961 under, or by virtue of, the provisions of this chapter or any other Act, the burden shall be upon the person or party claiming that such loss occurred, to establish such claim by a preponderance of the evidence. Any person who commits or performs, or who has committed or performed, any act of expatriation under the provisions of this chapter or any other Act shall be presumed to have done so voluntarily, but such presumption may be rebutted upon a showing, by a preponderance of the evidence, that the act or acts committed or performed were not done voluntarily.