Well, that’s an argument they should have made! I think it’s extremely charitable to assume this is why, though, when every indication points to code-audit fearmongering.

But, you’re also forgetting that these virus scanners can also be vulnerabilities and exploits in themselves; i seem to remember one virus exploited a flaw in the compression code of a virus scanner to establish some type of malware. Just because something is a trade secret doesn’t exactly lessen the risk of it existing.