We don't need first principals thinking for every thing. Granted, this comment is a bit vague, so I don't know how exactly you were taught or which type of class we're talking about, but generally, you can accept some axioms in applied mathematics classes. If we're talking the bare minimum classes like in the article (for apparently a business degree), this is likely a general applied prob/stat. Things tend to get more in depth with more advanced pure mathematics courses.