The vulnerability was discovered six months later by a German software developer. If he hadn't discovered the vulnerability, maybe we would still not know about it. If someone had looked at the code sooner, maybe we would have known sooner. Open source means that more people examine the source code, but it is no guarantee that vulnerabilities will be found or, even if found, fixed properly. If the person who discovered the vulnerability was intent on breaking into systems, this would have played out much differently.
Back doors have the unfortunate property of being all or nothing.
It's like leaving your house key under the mat. If no one knows about it, it's pretty safe. If everyone knows about it, it makes your door lock useless. Borland certainly belongs in the doghouse for this one. The Interbase discovery is more common than you might think.