Ah, be careful here! As an idiom 'to pluck something out of the air' does indeed mean to recite information almost without thinking. However, in your example it refers to literally plucking (grabbing/taking) ) the corn dog from the air! The idiom has become a physical act as it is in reference to the corn dog.

When you pluck anything, it's a quick act. It's to yank something quickly, sort of without having time to see how it's done. Think of plucking eyebrows or plucking a chicken's feathers. When we use it as a saying, it's to imagine a magician doing a trick where he/she plucks into the air with their fingers, then suddenly producing something in their hand, such as a coin, flowers, or a dove. It's a saying to mean that it doesn't make sense where the object came from. We usually say in English that something was "plucked from thin air" as a saying to mean to produce something on the spot. Craig mentions that we say it to refer to "recite information almost without thinking", but this is not exactly true. It could be, but most times it refers to what is being done so suddenly without reason, as opposed to what is being said. I'll give some examples: "Suddenly she had a hat on, as if she plucked it out of thin air." - the act of her putting on a hat was fast. Also, the speaker didn't even know where the hat came from. The speaker could have turned away for a moment, then turned back and noticed her wearing it. "For days we complained that we didn't have a remote control for the TV. Today Gino was using one as if he just plucked it out of thin air." - This is to suggest that there was no explanation about where the remote was from, Gino was just seen using it as if it was just suddenly there and available. A mystery! "I asked you repeatedly if you had five dollars and you said no. Now all of a sudden you do? What did you do? Pluck it out of thin air?" - Again, an object that may have seemingly not been there or available suddenly is. Money these days is a lot of times electronic, so even though the money may or may not be a tangible object, it is still an item that is available for use that is suddenly there without explanation. An example of saying something from out of nowhere: "I plucked that idea out of thin air." - to produce an idea without thinking, quickly.