The formula at issue is one incarnate nature of God the Word. Chalcedon's opponents held it; Chalcedon defined two natures in one person; and Christendom divided over the difference for fifteen centuries.
The difficulty is that the Greek word rendered nature was doing two jobs. In one usage it means something like a concrete individual existent, in which case saying Christ has one nature is a way of saying he is one being and not two, which Chalcedon also wanted to say. In another it means something like a kind or a set of properties, in which case saying Christ has one nature suggests his humanity has been absorbed into his divinity, which Chalcedon's supporters heard and rejected. The formula the Oriental Orthodox churches use came to them from Cyril of Alexandria, who was a saint on both sides of the argument.
From the later twentieth century, theologians from the Oriental Orthodox and Eastern Orthodox churches met in a series of unofficial and then official consultations and produced agreed statements concluding that the two families confess the same faith in Christ in different formulae, and that the anathemas of the past were directed at positions neither side actually holds. Parallel agreements were reached with the Catholic Church.
The formal separation stands. Agreed statements are not union, reception of them has been uneven, and there are jurisdictional and canonical questions the christological agreement does not touch. The atlas records the agreement and the continuing separation together, because both are true.