That's not how that works. An order can be legal whether these attacks are legal, extra-legal, or illegal (they're extra-legal or legal). Congress has given the president power to perform some military actions without their approval. Their approval is needed to declare war, and they're required to be notified of the president taking certain actions (which they weren't at the start, making what the president did illegal, but not the soldiers).

This isn't defending the actions. This is only a statement of fact, letting you and others know you don't know what the fuck you're talking about when you use this saying. Yes, they can (in fact, they must) refuse illegal orders, but that doesn't mean what you're implying.