I am always amazed how people start startups in areas they have absolutely no idea about, and build things for imaginary users that do not exist. Well guess that is what a big co "new products team" teaches you...

Not sure what you mean by imaginary users, he did speak to relevant people for his mental health startup venture. And while I agree it's magnitudes riskier to start in a domain that you're unfamiliar with, there are plenty who do so, and become very successful.