Okay, Diane, I think maybe I understand your point. Let me know if I get this right, or if I missed something.

Forester is trying to show that the cyclist-inferiority phobia exists.
As part of doing this, he explains how phobia is defined and assessed in the DSM. I don't think you have a problem with that part of his essay. As part of this, he cites the diagnostic criteria for Simple Phobia, including:

B: During some phase of the disturbance, exposure to the specific phobic stimulus (or stimuli) almost invariably provokes an immediate anxiety response.

Then he takes you through each criterium. For B, he states:

Consider Diagnostic Criterion B:

Does exposure to motor traffic from behind, while riding a bicycle, almost invariably provoke an immediate anxiety response. Certainly it does, and even contemplating the exposure of others to the stimulus causes anxiety response, as when people organize political committees to obtain protection from the condition that they fear.

If I understand your position, it is that he simply asserts, "Certainly it [exposure to motor traffic from behind] does [almost invariably provoke an immediate anxiety response]", without explaining why, or how he knows that it does, or even how the anxiety manifests itself. Apparently, he thinks it's so obvious that exposure to motor traffic from behind provokes anxiety that it is self-evident, and not requiring any further explanation or evidence.

If that's what you're trying to say, okay, that's a fair criticism, but it's hardly a debunking. In order to debunk it, you have have to go further and show that exposure to motor traffic from behind does not almost invariably provoke an immediate anxiety response. Without showing this, the most you can say is that there is insufficient evidence to be persuaded either way.