Not at all: that's the bug. It's not properly verifying that the other-end of a TLS session is the entity able to sign with the certificate-declared private-key.
Most likely hrrsn was referring to code-signing keys. Even if you can successfully MITM a software update connection, iOS won't run your trojan unless it's got a valid signature.
Of course the jailbreaking community knows well that there have been many ways around that...
