The most simple explanation of complete absence of the Parenthetical text (Comma) in the early Syriac manuscripts is because it never was in the earlier Syriac manuscripts - ever. 👍
Appealing to a Latin Pseudonymous Paratext that first appears anonymously in a sixth century A.D. Vulgate manuscript, which doesn't even have the said (commented on - matching) Parenthetical text within the same manuscript (i.e. in the Scripture text itself 👀), is really grasping at straws.
Actually, the lack of the text in Codex Fuldensis is essentially a verification of what is writing in the Prologue. There was a tendency to drop the heavenly witnesses. The Prologue and the text were transmitted separately, and the text was subject to the omission.
And I tend to agree the heavenly witnesses was not in the earlier Syriac manuscripts.
Again, appealing to a Latin variant text (actually variant within a variant of a variant of a variant text) as if it somehow that changes the evidence for the actual/earliest New Testament text Syriac manuscripts which date earlier than the earliest manuscript for this council.
What date are you ascribing to the Peshitta manuscripts?
If you accept the modern view of c. AD 400 your writing does not make sense.
The Council of Carthage points to a long history of copying the heavenly witnesses verse, as does the Vulgate Prologue. Which thus supports the Cyprian usage, and others.