Firstly, good psychologists are well aware that hypothesis testing provides nothing but the probability of the results occurring by chance, and that it has little to do with truth value. Secondly, based on your vague description, you came to your conclusion based on really weak observations and reading of the actual material. Way to show you are objective and able to weigh evidence. Thirdly, the actual rigor that goes into theories for cognitive psychology, behaviorism, even social psychology is far more complex than what is presented in an introductory class. Indeed, any good intro class will go over the action potential in great detail to hammer home the idea of monism. However, a complete understanding of brain structure, neurochemistry and electrophysiology is not necessary to develop fairly good models for social, behavioral, and cognitive processes. It doesn't hurt, but it is not the end all.

To lump clinical psychology and psychiatry together, and yet not differentiate clinical psychology from experimental psychology is ignorant.

Ultimately, experimental psychology, although not having as precise an approximation of evidence, as say physics, it does use the scientific method and ensures that the findings are transparent and open to the public.