To me the hilarious thing in all of this is that the verse in question [knowing this first] has nothing at all to do with interpreting the scripture. That's a bad translation. Wierwille comes SO CLOSE to revealing this, but it would undermine his larger point, so he lets the bad translation stand instead of out and out correcting it.
However, if you apply the keys to How the Bible Interprets Itself, you're left to conclude that this verse does not illustrate what PFAL uses it to illustrate. This verse is talking about the ORIGIN of scripture, not about the reader trying to understand it.
So the whole doctrine of "private interpretation" is misleading. The Bible never says to avoid it [because it's the world's least necessary instruction. You never saw Stan Lee worrying about people privately interpreting Spider-Man. The Bible assumes its meaning to be clear, and the section of II Peter 1 containing this instruction is telling its readers that "we" [I no longer include myself in that pronoun, so, y'all] are not following cleverly devised fables dreamed up by man, but real doctrines revealed by God.
That is, of course, if you still believe Peter wrote II Peter, but that's another can of worms and off-topic here. Here we assume for the sake of this discussion that Peter wrote his epistles, and we discuss accordingly. In that vein, Peter was not talking about the meaning of scripture. He was discussing its origin.