Hmm interesting.
Thanks for posting this. I'm happy to hear this from a semi conservative guy.
Sad it's such a surprise when it happens in some ways.
I am not sure I totally agree with it though. I mean what he says is true, but I'm not sure totally true.
In His word the Lord spoke the same as a verse in the Psalms. This does not mean that specific words to us are not very powerful and very needed and therefore should be very wanted. Think of when God spoke to Paul and said my grace is sufficient. That was not a Biblical quote. Or when He told Paul fear not I have many people in this city. Subjective personal specific prophesy should not be downplayed.
So I might even lightly say that either Piper was telling a partial truth from a partial word he received, he was adding to his word by his own Calvinistic bias, or his word wasn't actually true.