I don't see the consistency behind our actions in rejecting dialogue with the Assyrians while running after dialogues in seeking unity with the Chalcedonians.
Actually, while the EO's and the Church of the East have quite a bit in common (both accept the language of Chalcedon and the Tome of Leo) there are also differences between them. It is those differences which make the EO's Orthodox from our point of view, but not the Assyrians.
The EO's accept the Third Council, while the Church of the East doesn't.
Also, the EO's, since their "Fifth Council", have rejected Theodore of Mopsuestia, have stopped celebrating the feast day of Nestorius (some EO's used to celebrate it prior to that council) and have accepted the belief that God in the flesh suffered on the Cross. The Assyrians, on the other hand, still celebrate Theodore and Nestorius as saints and will not say that God suffered on the Cross. I think they prefer to say the Christ's human nature suffered on the Cross.
Also, although there are Assyrian theologians who accept the phrase "Mother of God," they will do so only if it is surrounded with qualifying language to make it clear that she is no more than the the Mother of Christ's human nature. The Tome of Leo is an example of this. It calls St. Mary "Mother of God," but evidently the other language in the Tome allows the Assyrians to believe it means something other than what we believe "Mother of God" means. The EO's on the other hand, since their Fifth Council, mean what we mean when they say "Mother of God" and use the phrase without reservation.
So you see, even though the EO's share the Church of the East's Christological language, the EO's actual beliefs about Christ are the same as ours, while the Church of the East is in some way different. Even my priest says the Assyrians are "heretics" and he is very ecumenical in his outlook.