Are you sure they meant that all the OO Churches were physically present at Ephesus 431? To say it was our last common council is not necessarily saying that we had actual representatives there. I know the Armenians did not have anyone there, and only accepted it after it happened.
I think that Ephesus 431 was our last common council in that it was the last one that all of the OO Churches officially accepted.
Later on, the Copts had Ephesus II and III, but the Armenians never officially accepted those. I don't think we objected to them, rather we just never had reason to officially accept them the way we did Ephesus 431. Likewise, I don't think the Copts ever officially accepted the Council of Dvin. I don't think they objected to it; they just didn't have reason to accept it.
Consequently, the last one that all of us have in common is Ephesus 431. It doesn't mean we all had people there. It just means that it is the last one we all officially accepted.