I shall now address the arguments from the site to which you referred Huldah.
Note 1: 1 John 5:7-8 fits the immediate context; in fact, it is an indispensable component of the surrounding verses.
Metzger, in his Textual Commentary on the Greek New Testament, argues that "as regards intrinsic probability, the passage [The Johannine Comma] makes an awkward break in the sense."
[1] Upon close examination of the immediate context, however, one finds that this assertion is far from true. For example, if the Comma is omitted, verse 6 and verse 8 are thrown together, "which gives a very bald, awkward, and meaningless repetition of the Spirit's witness twice in immediate succession."
[2] Furthermore, the omission causes the concluding phrase of verse 8 (and these three agree in one) to contain an unintelligible reference.
[3] What is "that one" (to en) to which "these three" are said to agree? In other words, "that one" in verse 8 which designates One to whom the reader has already been introduced does not have antecedent presence in the passage."
Let verse 7 stand, and all is clear, and the three earthly witnesses testify to that aforementioned unity which the Father, Word, and Spirit constitute."[4]
The passage makes absolutely no sense if the Comma is omitted.The phrase "in earth" in verse 8 as well as the entire ninth verse would also have to be knocked out to regain the sense because both infer that the "witness of God," as promulgated in the Comma, has already been introduced.
This is merely someone's personal opinion. It presents no new ideas and offers no grammatical argument nor any legitimate textual basis for its conclusion. It is, therefore, essentially worthless.
I advise you to read the footnote in the New English Bible (available
here.) The translators are Trinitarians, but agree that the gloss is totally insupportable.