My understanding is he publicly renounced his citizenship more than once. While that is not the same as formally renouncing it, it's good enough for me and the vast majority of Americans.
Any American who goes off and joins up with terrorists to plot, plan and encourage attacks on the US effectively makes them an enemy combatant open to US military attack. Sitting around trying to pick out the idiots that had/have US citizenship is to live in a world of theory and fantasy. The war on terror is just that, a war - not a police action.
Thanks. I was just looking for a source because I hadn’t heard that before. But you’re right, according to statute only a formal revocation of citizenship is legally recognized.