In Fate terms, it's not necessarily clear that it is "the very act of attacking that creates the advantage" because not everything that cosmetically looks like an attack will necessarily be one in mechanical terms. I mean, to take a simple example, pulling the trigger on a gun could be an Overcome, Create Advantage, or Attack action depending on where it was pointed in the fiction and what the player was actually trying to achieve right at that moment, despite the fact that it'd go "bang" in exactly the same way each time...Yeah, but it's noteworthy that it's the very act of attacking that creates the advantage in the clip. And you're right: Robin is forced to parry. If he did anything else but focus on parrying, the next attack would hit him.
Never mind that once one dips a bit deeper into the actual conflict rules, it turns out that temporary boosts that pop up on certain attack and defense results are in fact a thing there on top of everything else.