My thesis, according to the OP, is that there are fundamentalist (strident, fanatic, or dogmatic) atheists making claims about the real world that are demonstrably unreasonable, unrealistic, and illogical.
It wouldn't take long to find atheists that fit this description. A scan of a few articles at HuffPo would probably turn up a few. I wouldn't doubt that you could find an atheist that praises the curative powers of homeopathy.
Nevertheless, under a false assumption that the adoption of atheism automatically makes one reasonable, realistic, and logical (or something like that), these fundamentalist atheists maintain their positions to be reasonable, realistic, and logical.
I really enjoy reading the articles at Pharyngula. This isn't because I agree with everthing that PZ says. I just find it entertaining. PZ walks a fine line between atheism and the strident anti-theist you are talking about. Sometimes he brazenly blows right past the line (which is why I find it entertaining). At the same time, he can be quite reasonable and even admit where his arguments could be flawed.
Can atheists be unreasonable and completely wrong on certain subjects? Well, yeah. Atheists are also human.
My main question is: If these claims are so ridiculous, why do these people make and cling to them? What is the mindset(s) fostering the irrationality of irrational (fundamentalist, strident, fanatic, or dogmatic) atheism?
Ego, emotional investment, defensiveness, . . . you know, our human flaws. Like anything, each and every claim needs to be judged by the evidence, not by who says it. Einstein got Relativity right, but his dogmatic position with respect to a Steady State universe did more harm than good as one example.