This isn't security through obscurity, unless the DRM implementation being a secret actually does provide security. I doubt it does, beyond the fact that an audit of the source could probably find a load of security issues.
I don't necessarily disagree. But how, other than through obscurity, does HTML5 DRM inhibit copying, given that the client possesses the decryption key? (Let's assume the would-be attackers aren't dissuaded by any laws that might apply.)
Of course an audit of OpenSSL would do the same.