I don't know how big of a "secret" it actually is -- I think it's just that most people make assumptions about what HR does and why they exist. It's less that the information is hidden and more that people just don't bother looking for it.
My wife is an attorney who works in HR and labor relations. HR existing to protect the company was one of the first things she learned in either an HR class or employment law class.
It's a "secret" because the manuals that HR produces contain explicit or implicit statements to the effect that HR is there to help the employee with things like benefits and conflicts with other employees or managers. Most people aren't ever caught up in a tangle in which HR's true purpose was exposed to them, so HR gets away with these little lies and maintain the illusion that they're there to help employees.
My wife is an attorney who works in HR and labor relations. HR existing to protect the company was one of the first things she learned in either an HR class or employment law class.