Armenia was not at Chalcedon. In 451, the year the council met, the Armenian church and nobility were fighting the Sasanian empire over an attempt to impose Zoroastrianism, and the battle of Avarayr was fought that same year. Nobody could be spared for a council in Bithynia, and by the time the definition arrived it came as a foreign document already carrying imperial politics with it.
What arrived, moreover, arrived through Syrian channels and in a form shaped by the churches that had already rejected it. Armenian bishops were reading Chalcedon in translation, presented by people who told them it divided Christ into two, and there was no Greek-speaking Armenian hierarchy in a position to argue otherwise. A council at Dvin in 506 took a position against it. The council of 555, under the Catholicos Nerses II, made the rejection formal and permanent, with a confession of faith and canons preserved in the Armenian collection known as the Book of Letters.
It has never been reversed. The Armenian Apostolic Church stands with the Coptic, Syriac and Ethiopian churches in the family usually called Oriental Orthodox, defined precisely by the non-reception of this one council.
The lesson the case teaches is about how councils actually work. Chalcedon was as ecumenical as a council gets: imperially convened, patriarchally attended, doctrinally decisive. It still did not bind a church that was not in the room and had other things to do that year.