There are two different stories of how the hymn came about. The OO say it was sung at Christ's burial, the EO say it was heard by the inhabitants of Constantinople during a great earthquake in the early 5th Century. In both cases, the origin of the hymn is not Syrian.
I would say this is a fair way of looking at the Trisagion. No one knows the intent of the Trisagion. Some say it was Christological and some say it was a Trinitarian hymn. But I think it would be polemical to say one took it and changed it without showing us proof.