The author does not rule out the evidence presented in the scriptures (read first paragraph of article).
Scripture is what a believer considers scripture. It's tangential on an a priori human acceptance of something as scripture. That's why scripture cannot be a proof.
So anything can be scripture. You can go around and believe any two bit book is inspired - even one written in 1830 - is the word of God. God preserved His Word.
Then he is contradicting himself (which he does anyway), because he writes "We do not need to presuppose that the New Testament is infallible, or divinely inspired or even true." He merely argues that we need to axcknolwedge the existence of the New Testament.
So anything can be scripture
That's correct. To you Koran may not be scripture because you choose not to believe it is. To a Jews, the New Testament and a Book of Mormon is as good as toilet paper. To a Christian, the book of Mormon and in fact the whole LDS is a cult and its 'holy' books as good as something satanic. To a Hindu, all the monotheistic scriptures are interesting but incomplete.
You can go around and believe any two bit book is inspired - even one written in 1830 - is the word of God. God preserved His Word
That's pretty much how it works. How do you know the books are 'inspired?' How do you know what God is? Simple: whatever you make it to be. There were hundreds of versions of the current NT and hundreds of additional manuscripts, all pretending to be "inspired" apostolic writings, such as the Apocalypse of Peter, etc. which never made it into the agreed-upo Bible.
In every case, the book selection was a human choice, or so it seems. None of the NT authors says he were guided by the Holy Spirit in his writings.