Who even implied that?
Seriously, that's just stupid.
Can constitutional powers be misused? You bet.
And so can contra-constitutional powers — actually it can be harder to fight extra-constitutional powers, precisely because they're outside the confines of the Constitution… and oft they are legally justified on precedent, elevating precedent above the Constitution.
It's implied in any claim that American citizens cannot be killed by US armed forces overseas.
For this "right" to be implemented, fairly obviously a hearing must be held to determine citizenship status before a soldier returns fire.
If US citizens aren't to be targeted, how do you know whether a possible target is a citizen without such a hearing?
BTW, the above is an example of trying to carry an argument to its logical conclusion to demonstrate its flaws. However, I see no reason why those planning and launching attacks on us should be treated differently based on citizenship status. If we catch em, we can try them for treason in addition to any other crimes, but that's about the sole distinction I can see.
I agree with you about extra-constitutional powers being dangerous.
In 1901 the Supremes ruled that the Constitution does not always necessarily follow the flag, in Downes v. Bidwell, even when the government is in control of territory. How much less does our Constitution apply in foreign countries?