I can GUARANTEE as a former US fighter pilot he probably knows some stuff and probably signed a bunch of stuff that he wouldnt teach tactics of the US military to our enemies... If he isnt being charged with treason, I can only guess to what he is being charged with.
My GUESS is that its not necessarily the USGov going after him, but possibly the US Mil via UCMJ and because he was living in Australia they have to have the FBI go get him. If its US Mil going after the guy I am pretty sure he signed his rights away when he went in and probably, again if he is "selling" knowledge gained via his service in the Marines, that clause in his "contract" is still in force. I dont believe the US Mil/UCMJ has to disclose a single thing about what they are doing like a regular DOJ or other civilian type law enforcement organization would possibly have to do.
So no, im not really bothered that a "click bait" type news article says nothing except everything was under seal. It may or may not ever come out what he is charged with. This is also taking place in Australia where there laws are possibly a bit different than ours as far as privacy, or lack there of, goes.