Even as I was reading it, I thought to myself, "this isn't typical of Matthew Henry." Thanks for clarifying the authorship although there is some dispute I understand as to exactly which "Reynolds" did the 1 John commentary.Wrong!
Only true in the sense that textual criticism today is a false science.
John Reynolds (1667-1727) of Shrewsbury did this section of the commentary, after Matthew Henry (1662-1714) passed.
His full section is very informative and quite excellent, worth a careful read even today.
1759 edition
https://books.google.com/books?id=w4NaAAAAYAAJ&pg=PA571
CCEL
https://books.google.com/books?id=_LooN0wXs2AC&pg=PT2004
https://www.biblestudytools.com/commentaries/matthew-henry-complete/1-john/5.html
As for the argument, possibly the worst I have seen:
"Cyprian, cites John's words, agreeably to the Greek manuscripts and the ancient versions, thus: Ait enim Johannes de Domino nostro in epistolâ nos docens, Hic es qui venit per aquam et sanguinem, Jesus Christus, non in aquâ tantum, sed in aquâ et sanguine; et Spiritus est qui testimonium perhibet, quia Spiritus est veritas; quia tres testimonium perhibent, Spiritus et aqua et sanguis, et isti tres in unum sunt—For John, in his epistle, says concerning our Lord, This is he, Jesus Christ, who came by water and blood, not in water only, but in water and blood; and it is the Spirit that bears witness, because the Spirit is truth; for there are three that bear witness, the Spirit, the water, and the blood, and these three agree in one. If all the Greek manuscripts and ancient versions say concerning the Spirit, the water, and the blood, that in unum sunt—they agree in one, then it was not of them that Cyprian spoke, whatever variety there might be in the copies in his time, when he said it is written, unum sunt—they are one."
If I am not mistaken, this quotation comes from an anonymous author and not Cyprian.