If one person knows about it, then the network is insecure to him but secure to everyone else. The vulnerability exists, whether or not anyone knows about it. Publishing a vulnerability does not cause the network to be insecure. To claim that would be confusing knowledge about a thing with the thing itself. Publishing increases the likelihood that an attacker will use the vulnerability, but not the severity of the vulnerability.
Publishing also increases the likelihood that people can defend against the vulnerability.
Just as an attacker can't exploit a vulnerability he does not know about, a defender can't protect against a vulnerability he does not know about. So if keeping vulnerabilities secret increases security, it does so in a fragile way. Keeping vulnerabilities secret only works as long as they remain secret-but everything about information works toward spreading information. Sometimes secrets are re-derived by someone else. And once a secret is out, it can never be put back.