Tell that to a marketing agency.

Actually, marketers are well aware that statistics don't tell them HOW people work; they only tell them what gets the most response. Knowing that a "Johnson box" imrpoves results on mailing A to list B only suggests that it might work with mailing C to list D; it does not tell you how or why it worked, nor does it give you any real way to explain the result when it doesn't work.

Most marketing is useful folklore; good theories in marketing are very few and far between. Even the best teachers of marketing rarely rise to strong theories; the ones that do are mostly either borrowing their models from NLP and hypnosis, or reinventing them.

The quote is not about whether statistics might tell you something useful about people in general, it's about understanding HOW a specific individual is doing what they're doing. A statistical model can suggest paths to investigate, but it can't tell you what's actually going on in a specific case.

IOW, it's an epic FAIL for a mechanic to "rely on statistics as a way of understanding how" your car works (or doesn't) instead of actually observing what a specific car is (or is not) doing.