Smoky is correct here. The example I will use is Fillipinos that have served in the US Navy since prior to WWII. They are active duty, take the same oath, but are not Green card holders or Citizens (though most end up becoming citizens)
Other nationalities as well - I just had occasion to interact with the Filipinos. Usually in ratings like mess cook, etc.
They could definitely be charged with Treason though not US citizens.
Perhaps because 0vomit impersonated a US citizen and acted in that capacity, he could be charged with treason. From what I read on the research threads, there was no naturalization or green card issued.