Much like Overgrown Estate is a VP card because it says so, Donald could scribble "Attack" next to "Action" onto Scout if he wanted to, and then it would trigger Horse Traders and Urchin, even though the only "attack" effect is the severe demoralization your opponent feels when getting trounced by a well-tuned Scouts deck. He should consider that, too... triggering Reactions with null attacks would help bring Scout's overwhelming power back in line, ever-so-slightly.
But yeah... specific wording: "...plays an attack card" vs "...attacks you." Following the letter of the law, reaction is resolved when a card with type "Attack" is played. It's even theoretically possible for a treasure to have Attack as a sub-type.