“But you’re missing words!”
“But everyone knows what they are.”
And there, you have the essentials of a grammatical ellipse: what it is, and why it’s permissible, according to CMOS17 [5.229]—the 17th edition of the Chicago Manual of Style, perhaps the most widespread guide to editing and publishing.
Unlike a regular ellipse—the three little dots that indicate words that are missing or faltering—there is no typographic symbol to indicate that words are missing in such an elliptical sentence. We just know.
“Later” might be a grammatical ellipse when we understand it to mean “We’ll talk later.”
While a stickler for formal prose might want to add “dealt in” to the second clause of “She dealt in AP, he APA,” CMOS17 explains that the omission is not grammatically incorrect. It is a type of grammatical construction particularly appropriate to speech—an elliptical sentence.
Elisions make sentences easier to understand when speaking, CMOS17 says. We use them most when answering questions: “Where did you read it?” “Copyediting,” we answer, rather than responding, “I read it in the Copyediting Newsletter.”
When people share a common language, and understand the context, they are able to mentally replace the omitted words. It’s the principle of recoverability—a term which makes logical sense but that Google is not overly familiar with outside of economic contexts. We use these constructions naturally when making commands: “Give it!” (Give the tuggy to me, puppy.)
So then, the editor’s job is not to mark such constructions as incorrect, but to determine if such an elision will be understood by the reader, and if the context does not demand a more formal, wordy construction. Indeed, when writing dialogue, filling in such elisions completely change the voice of the character, and that is a very serious consideration.