No, that is warning about adding to or remove from the prophecy of Rev. It does not affect any prophecies God reveals to new prophets. John was not shown everything. No prophet is.
Your logic is unsound. To fulfill the prophecy A with new prophecy B is by any measure adding to the prophecy A.
One last point. Consider 2 verses of Revelation. All speaks of witnesses. Coincidence? Or do arguments of spiritual anthropomorphism still hold sway.
Revelation 1:2
Who bare record [3140] of the word of God, and of the testimony [3141] of Jesus Christ, and of all things that he saw.
Revelation 11:3 ►
And I will give power unto my two witnesses[3144], and they shall prophesy a thousand two hundred and threescore days, clothed in sackcloth.
strong's 3141. marturia μαρτυρία, ας, ἡ
Part of Speech: Noun, Feminine
Usage: witness, evidence, testimony, reputation.
strong's 3140. martureó μαρτυρέω
Part of Speech: Verb
Usage: I witness, bear witness, give evidence, testify, give a good report.
strong's 3144. martus μάρτυς, υρος, ὁ
Part of Speech: Noun, Masculine
Usage: a witness; an eye- or ear-witness.
I have reasoned all I care to, I leave the last words to you. Have a good day.