It is a very interesting article. I remember we discussed this matter in detail at the Bereans forum. I would like to mention here some of the conclusions I made when considered this topic.
1. There is no such thing as "sinful nature" or "sinful substance". Sin is not a substance, but a defection of will (according to Augustine). When Lucifer fell, he did not have any sinful nature or substance enter him. Sin is not a matter of substance, but of volition. When Satan seduced man, he did not inject into him substance - he effected his volition. Therefore, the fall did not added some other substance to human nature, but deformed human nature. We can compare the fall with the untuned piano. Piano originally was tuned to produce good music. But then it was untuned. It does not mean that some kind of nature entered it - it was simply disharmonized. Man was created a harmonious being. But fall disharmonized him.
2. The flesh in the Bible is ususally defined as a pattern of living, and not as some kind of substance. This pattern of living is shaped by the desires of our heart and body that are independent of and contrary to God.
3. Charles Fynney said that the fall of man was moral. He said that should it have been physical, man would not be subject to be judged by the law of God. If we sin against our will being compelled by some kind of nature, we are not to be judged. Only voluntary actions are judged.
I think that these points are enough to start good discussion.
