Aren't the Armenian episcopal mitres influenced by western mitres? Western influence is no bad thing, and that was never what I was trying to say.
That's actually what makes me think the altars are not from the Catholics. We did borrow several things from them during the Crusades when we had a lot of contact with them and the Armenians in Cilicia actually united with them for a while.
However, whenever I have read anything on this, or heard anyone talk about it, no one mentions that one of the things we borrowed was the shape of the altar. Other things will be listed, like the shape of the bishop's mitre, and the confession read by the priest at the beginning of the liturgy. But I've never heard anyone claim that the shape of our altars came from them.
Also, I don't think the Syriac Church was influenced by the Catholics the way we were, or that they would have borrowed something like this from us. To the extent that you find things in common between the Armenian and Syriac Churches, it is due to a very early connection going back before the fourth century, and the influence was from the Syriac Church going to the Armenians.