You are misinterpreting 'obscurity' to as much of a degree as the grandparent post, though in a different way. The 'obscurity' that the adage speaks of is meant to relae to the system you're using. ROT13 is security through obscurity: as soon as your adversary knows you're using it, your system is broken. RSA is not security through obscurity: you can advertise that you use it (indeed, that's fundamentally a part of public key crypto), and still be safe.