I don’t know how else to describe a scenario where Antiochians and Syriacs have full access to the sacraments at each other’s churches, are prohibited from converting, can be the godparents of children in either church (so an Antiochian can have Syriac Orthodox godparents), and where the churches have recognized each other’s legitimacy and laid down protocols for episcopal concelebration. It is Eucharistic communion which determines the status of ecumenical relations, and the only thing that as far as I know has not happened is concelebration, but since the laity can attend either church, and since both are recognized as legitimate according to the agreement, this means that as far as the laity is concerned, it is as if the Antiochian Orthodox and the Syriac Orthodox are in communion.
Now, whether or not this applies to Eastern Orthodoxy is a question of ecclesiology. According to the ecclesiology I see many Greek Orthodox using, the Antiochians are no longer Eastern Orthodox, if the principles of their ecclesiology are objectively applied. Certainly many Constantinopolitan Orthodox and members of the OCU have claimed the MP is no longer Eastern Orthodox, but the conditions that characterize the MP also apply to Antioch, to a still greater extent.
Your first statement is clearly not true. I have copied the 4 provisions of the letter agreement below. There is no "...full access to the sacraments...." 6. applies only if there are both bishops present for a funeral, baptism, or matrimony, and merely says who presides. 7. prohibits mixed bishops concelebrating Divine Liturgy, while 8. extends that to priests.
The tricky one is 9. where a priest from one can celebrate services including Divine Liturgy and matrimony, for the other,
if there is no priest of the other. I have some questions, but clearly yes there is literally communion between them. I still relate this as an accommodation between two churches in a hostile land. If there is a requirement that there is a church building - a physical church - with no priest, but with a community, that is even more support that this is an accommodation.
I do not know what happens in practice, but the letter is written so as to bar "...the laity can attend either church (meaning for communion)...". The letter gives only one situation where that is allowed.
Letter provisions:
6. If bishops of the two Churches participate at a holy baptism or funeral service, the one belonging to the Church of the baptized or deceased will preside. In case of a holy matrimony service, the bishop of the bridegroom's Church will preside.
7. The above mentioned is not applicable to the concelebration in the Divine Liturgy.
8. What applies to bishops equally applies to the priests of both Churches.
9. In localities where there is only one priest, from either Church, he will celebrate services for the faithful of both Churches, including the Divine Liturgy, pastoral duties, and holy matrimony. He will keep an independent record for each Church and transmit that of the sister Church to its authorities.