As do those who refuse to get on airplanes due to their fear of flying.
As do those who refuse to walk into caves due to their fear of closed spaces.
Etc., etc.

So what, you ask? Forester's point is that if the reason someone does not do something is because of a phobia (an irrational fear that is inconsistent with reasonable risk analysis), then only a certain course of action (repeated successful exposure to the feared something) can help them. The relevance to cycling advocacy is that if the reason someone does not ride his bicycle much, or as much, is because of the "cyclist inferiority phobia", then only a certain course of action (repeated successful exposure to riding in traffic) can help them.

This is why the issue of whether the cyclist inferiority phobia exists is relevant to cycling advocacy.