Not quite correct.
The Protocol II addendum to the Geneva Convention does cover them. However, the US has never ratified that additional protocol, therefore, it does not apply to us.
Their families, however, are covered by international law to which we are a signatory. They can be collateral damage, but combatants must take necessary precautions, and they may not be deliberately targeted.
I wouldn’t disagree with the families, but beyond that, terrorist are just flapping in the wind.
Even if the Geneva Convention were an iron-clad agreement to protect terrorists, if I were president I would ignore it.
If a guy can walk up beside me without waring on a public street and stab me to death, I don’t care what the Geneva Convention says or doesn’t say. It’s preposterous on the face of it.
If a terrorist may have information leading to the prevention of a weapon of mass destruction going off in a highly populated area, you can bet your hind end the GC would be the last thing on my or any other sane person.
I believe that Trump was expressing the same frustration we all do. I do not expect him to break international law on a whim. If it comes down to your or my families lives, I hope he does.
The Geneva Convention allows for US waterboarding and worse:
“The Conventions apply to a signatory nation even if the opposing nation is not a signatory, but only if the opposing nation “accepts and applies the provisions” of the Conventions.[12]”
https://en.m.wikipedia.org/wiki/Geneva_Conventions