My understanding of shibboleth is that it refers to the primary pronunciation identifying the speaker (which is how it was used biblically).

Those are also interesting but it feels a bit more special to me when it's a secondary speech characteristic because it feels a bit more like a clue in a detective novel -- it almost feels like you can hear inside the author's head and how they vocalise while they write.

In this case, "missle" is a malapropism that is only possible if you don't pronounce the second "i", and so is a secondary "speech" characteristic.