I think it can be inferred that a Counterattack, and an Opportunity Attack are not attacks simply because they are keywords. They are each their own specific "things" separate and distinct from one another.
I don't think I disagree with you as such, but I do want to probe the question of "what is an attack?" a bit more.
It sounds like you are saying that nothing is an attack other than what the rules explicitly state is an attack. Based on that, you are saying that an opportunity attack is not an attack, as per your quote above. Right?
"Reaction: Opportunity Attack
Cost: Reacting model gains 1 fatigue. Trigger: After an enemy model declares a Move Action while in the reacting model’s engagement range. Effect: Roll ATK +1 bonus success versus opposing DEF of that enemy model."
It doesn't say anywhere that it is an attack, so it is not an attack.
So would you say that special rules that refer to attacks never apply when it comes to opportunity attacks?