Remind me again when there was a declaration of war against the Islamic State of Iraq and Syria? Because we're currently helping them in Yemen.
Bearing arms (if she did) in Syria, against Kurdish allies of the US (until Trump hung them out to dry) is kind of a stretch. We did have US military personnel in Syria at the time, but that's a low bar. We have US military personnel in a lot of places.
As far as the tweets saying to kill Americans, 1. she says she didn't make them, 2. I can't actually find them, but I'm pretty sure that it would require some actual immediacy, not "Fellow Muslims in the US, Rise Up!!".
Mean Tweets are not treason. She's a *expletive deleted*tard, and deserves to hang out in a Syrian prison camp for a good long time, but I just don't see treason for *expletive deleted*ing some goatherds and making tweets, honestly I'm not sure I would even if it could be proven someone joined because of her social media. I admit I'm kinda jaded but calls for execution strike me as more of the Bush era "You're with us or an Enemy!!" rhetoric circa 2006 or so. The whole conflict was more nuanced and foggy than that. Find me evidence she was making IED that were placed in Iraq for US convoys, sure you have a point. Syria circa 2014 was more like siding against our catspaw in the proxy war. I could be wrong though.
And that's ignoring the whole point that apparently as far as the US government appears to be concerned, she's not a US citizen and never has been. Which would make it difficult to commit treason.