You should probably try to understand the context, or at least the definitions, before you go tossing around phrases like "one of the most profoundly stupid statements ever uttered by an economist."
You can study human behavior in a lab, at least on a statistical basis and within carefully designated boundaries. Outside those boundaries verification and falsification become rather difficult. For example, your experiments tend to become invalidated if the subject knows that they're being studied, and what the hypothesis is, since humans are adaptive and this knowledge may change their behavior. Also, human behavior is influenced by hidden variables which cannot be directly measured, in a lab setting or otherwise. What is studied in a lab are our purely physical limitations, and our psychological inclinations—neither of which is properly described as part of the study of human action per se. The former determines the scope of our actions, and the latter determines our preferences. The study of human action, however, is the study of how we go about satisfying those preferences; in other words, the study of choices. It is thus a general predictive model of human decision-making which Mises is placing outside the scope of verification and falsification by laboratory experiment, and rightly so.
If you start from the position that choices can be practically reduced to nothing more than measurable impulses in the brain then you have no use for the study of human action. However, even for the purest materialist there remains the fact that we do not currently have the capacity to make that approach practical. Just as chemistry remains a useful and independent avenue of study apart from physics, despite the fact that chemistry is in essence nothing more than the statistical study of certain classes of interaction between fundamental particles governed entirely by physical laws—because we currently lack the capacity and/or knowledge to make the leap from those fundamental particles to the overall properties of specific molecules and the reactions between them—so does praxeology (the study of human action) remain useful on its own merits, independent of the related scientific studies of physics, psychology, sociology, and the like.