One who voluntarily serves as a commissioned or non-commissioned officer in the military of a country not engaged in hostilities with the United States will lose one’s U.S. citizenship only if one intended to relinquish U.S. citizenship when he/she served in the armed forces of a foreign state.
What’s that got to do with it? She was involved with terrorist organization hostile to America, not the army of a nation, so her intent has nothing to do with it.
There is a report of a video showing her burning her US passport and stating she will never need it again. If true, that shows intent to renounce her US citizenship, (if she legally was a US citizen since it is not clear if she was born to a foreign diplomat which would make her ineligible for US citizenship).