Keeping implementation details secret DOES make it harder for a hacker to compromise them. When used as a defence on top of a decent security infrastructure. "Security through obscurity" is when a company only uses the secrecy as a defence. This is not true:

> they're all reliant on security by obscurity

They're generally reliant on some secure and proven methods of security, with a layer of design obscurity over the top (and in practice as others have pointed out, they don't keep the design secret for security reasons, they do it for commercial ones).