It's important to remember the context in which Peter was writing. Which church he was writing to is never specified, but he's talking about his desire that even after "this my Tabernacle" (i.e. the earthly temple of his body), the church would stand strong in proper doctrine and understanding of the gospel and live lives pleasing to God, as Peter himself was about to be martyred in Rome. He wanted to remind the church of the value of the gospel they've been taught to help them stand firm, even after the deaths of the original leaders, because Christ crucified is bigger than any individual human leader.KJV 1611 wrote:For wee haue not followed cunningly deuised fables, when wee made knowen vnto you the power and comming of our Lord Iesus Christ, but were eye witnesses of his Maiestie. For hee receiued from God the Father, honour and glory, when there came such a voice to him from the excellent glory, This is my beloued Sonne in whom I am well pleased. And this voice which came from heauen wee heard, when we were with him in the holy mount. We haue also a more sure word of prophecie, whereunto yee doe well that ye take heede, as vnto a light that shineth in a darke place, vntill the day dawne, and the day starre arise in your hearts: Knowing this first, that no prophecy of the Scripture is of any priuate Interpretation: For the prophecie came not in olde time by the will of man: but holy men of God spake as they were moued by the holy Ghost.
The New Testament had not been written and compiled by the point in time at which Peter was writing (roughly the late 60s AD); when the apostles went to share the gospel, they had to rely on Old Testament prophecies about the Messiah to convince people that Christ had come. False teachers were already springing up by this point, and the apostles were trying to stop evil subversions of the gospel before they started to spread. One of those subversions was a direct attack on the legitimacy of the Old Testament prophecies—writings like the ones in Isaiah and Ezekiel and the Psalms didn't mean what the apostles said they meant. These false teachers argued that they were written by fallible men who were just interpreting things as they happened to them, so who knew that Jesus was the one to whom the Gospel actually referred? Maybe the original prophets were just writing personal anecdotes with all the bias therein, and furthermore, the apostles were taking advantage of it to lie to people, and Jesus wasn't actually the Messiah.
Peter here says in rebuttal that they weren't just repeating "cunningly deuised fables" when they gave their testimony of the gospel to the churches—the apostles hadn't just made this stuff up. The church could trust Peter's word on this, as with the word of the other apostles, because they "were eye witnesses of his Maiestie" when they saw the Transfiguration, and he describes very specific details about that event. Furthermore, the words of the prophets came "not in olde time by the will of man", but rather, "holy men of God spake as they were moued by the Holy Ghost". Personal anecdotes about historical events are based in interpretation and human bias, but the ancient Hebrew prophets weren't telling personal anecdotes; it was their job to directly communicate with God, and therefore what they wrote down was what God wanted them to.
In other words, the verse you are using does not seem to be talking about people's individual interpretations of Scripture as they try to follow God's word in the modern day. It's talking about the reliability of Old Testament prophecies, and the reliability of the testimony of the apostles. Of course it's not unheard of to interpret the verse the way you're presenting it, but looking at the passage as a whole and the circumstances of its writing, it seems implausible.
(Also, JK Rowling is a woman. ...That's not really as important as Scriptural interpretation, but it bears saying.)