Once they declared it a terrorist organization (which is the problematic side of everything), they can claim these are unlawful combatants and do not have any of the protections of the Geneva convention, like any other war on terror assassination.
So I don't think double tapping is a war crime, any more than bombing a car with terrorists in the first place and that doesn't seem to be regarded internationally as a war crime. However, they could have done better to highlight Venezuela actual involvement with terrorism (which is real but not enough for this) rather than magically declare them terrorists just to not go through Congress
If you need to look for loophole justifications to claim there was no war crime, there was probably a war crime.
That "unlawful combatant" designation was invented by the US as an excuse and has always stood on shaky legal grounds even in the US. Other Western countries don't support this legal construction. That being said, the double-tapping was ordinary murder, not a war crime. Every bombing of those ships could have been avoided by boarding them and presenting those drugs as evidence, as the Coast Guard normally does. But that would only have worked if there had been any evidence to start with...