The portion you quoted
Was to underline Christy’s words…
Though to be sure, there is technically a dispute between Catholicism and Protestantism. The former giving authority to the church and the latter to the Bible alone. I have also made it clear elsewhere that I do not think the Bible is sufficient for the definition of “Christianity” but for that there is also the decision of the ecumenical council of Nicea 325 AD.
As for “corrections” to the interpretation of the Bible by modern day “prophets” like you have in the LDS and JW, that is what puts them outside of Christianity. If these were simply how they chose to understand the Bible and the Nicean creed instead of as a correction to them then I don’t think there would be any separation. This is not to say they would not be criticized as the denominations of Christianity do criticize each other.
But to be Christian, the Bible has to be accepted as the word of God requiring no corrections whether you call them “correct interpretations” or additional revelations and scriptures. In Christianity (defined by the early ecumenical councils), the only thing with authority for the teaching of Christian beliefs is the text of the Bible alone.