Now we come to the crux of the argument:
"beliefs worthy of being called knowledge must submit to the tribunal of intersubjective, that is, publicly observable, evidence. Objectivity is only gained through intersubjectivity."First of all, this statement is arbitrary. There is no basis. Where is the author's authority for such a requirement? While it sounds nice, there is no guarantee that intersubjectivity will lead to objectivity. Nor is any reason given that intersubjectivity (the experience of many) is better than individual experience.
Now I agree that the experience of many is superior to the experience of one. But that is because I believe in sin. A singular sinner becomes great in his own eyes. He overlooks his sin - that is nature of sin. Multiple people can see sin in others, that is also part of sin (I focus on your sin, and overlook my own)!
There is also a core failure here. It is the assumption that many eyes make for perfection (or truth). The problem is that a core design defect will be present in all eyes.
It's like the Pentium FDIV bug. This bug was present in all the first generation parts. You could run your problem on one computer, or a thousand. They would all fail in this regard. You can actually get slightly different results using equivalent algorithms due to the vagaries of floating point arithmetic. For most problems, you can accumulate the results to get a better answer. But for a select set of inputs, the answer would be just way off.
Similarly, intersubjectivity ignores man's Fallen nature. The Bible says the mind of the natural man is the enemy of God, and dysfunctional. No number of Fallen men working together will ever find God, understand God, appreciate Him, etc.