In my reading today, I came across this phrase:
Pardon the mixed metaphor, but you’ve brought a knife to a gun fight. [Link]
The (longtime) columnist was saying the person was outmatched. I understand that "gunfight" for a conflict is a metaphor, but I don't understand why the entirety is a mixed metaphor.
A mixed metaphor is mixing parts of two metaphors to make a, well, not a known metaphor or one that's ridiculous, e.g. to talk a very long time without significant results can be expressed by saying,
talk until you're blue in the face/until your face turns blue
until the cows come home
Both are common metaphors. Mixing them would produce, say,
Talk until the cows turn blue.
I tried Googling variants of the supposedly mixed metaphor to see if there was a more established version. I could not find one.
There were references to the origin of the above idiom, my favorite (supported here as well) being from the movie, The Untouchables, wherein Sean Connery utters with contempt, "Isn't that just like a [racist for Italian]... brings a knife to a gun fight." (There were others but none earlier.)
The following, first appearing in print in 2008, might be considered a mixed metaphor:
Those who live by the sword get shot by those that don't.
But I can't understand the idiom in question as a mixed metaphor. Can someone tell me what I am missing?
Edited to add an example, and in case I was unclear: if it is a mixed metaphor, which are the two metaphors it's mixing?