Because its a hypothetical where the journalist didnt specify "unlawful order". Its a bs gocha question. There are hypothetical situations where it would be lawful, regardless of how likely.

War isn't actually illegal. I know it feels like it should be, but it really isn't. People say a war is "illegal" because I suppose they feel like it carries more weight than to just say it's "immoral". I guess it's somehow comforting to people to think that somehow some lawyers could prevent countries from going to war somehow.

How is bombing a wedding a lawful order?

The military is trained to be mindless sychophants.

It's technically not illegal to invade another country unprovoked. It's illegal to do war crimes though. You can invade a country and annex it without committing warcrimes.

Lawful /= you agree with it, basically.

To see why this is definitely within the powers the president has, swap "Canada" for "Vietnam", "Iraq" or "Iran". For more detail, you'd have to refer to the actual laws of war the US government recognises.