It's all about the context. I think.

Here's the rub: At the center of narratives/stories/rhetoric/whatever you want to call it, there is a speaker, a claim, and an audience. The speaker has a certain context and idea they want to transfer to their audience, and that speaker must also interact with circulating contexts, ideals, and expectations that exist within their audience and environment they choose to share their message. Within every interaction or shared message there’s a statement that is trying to do one (or all) of the following:

•          Entertain
•          Inform
•          Persuade
•          Educate

What I propose is this: Yes, Abbott is correct in saying that "All fiction can be profitably regarded as argument." However, I think the context surrounding the fiction can alter the appearance or acknowledgement of the fiction/argument by the audience. If the context or environment surrounding a piece of fiction is used purposely to confuse readers, then I can see how Abbott’s statement might be construed as incorrect. This is especially prevalent in pieces of fiction that are meant to redirect the reader, like Magic for Humans or Stranger Things. However, we have to keep in mind that if we as audience members go into a piece of fiction having some awareness that the fiction is meant to redirect us in some way, that awareness might dilute Abbott’s idea.

But I want to talk a bit about the word profitably for a minute. Abbott is saying in this case, argument propels fiction forward and gives the characters, narrator, narratee, and (dare I say) readers, more diversity than a piece of fiction/message that does not contain an argument. I think there's a reason that profitably is placed in Abbott's statement--he realizes that a story and piece of fiction cannot flourish without some form of conflict/argument or purpose to move the fiction along for audiences. Regardless of the intent (entertainment,informative, persuasion, education, or otherwise), without an argument or representation of an argument a piece of fiction and all parties within it remain static. This results in fiction that can likely become distant to its readers/audiences. Profitability is directly related to the complexity and movement of character/plot development. 

So, overall I do agree that fiction can be profitably regarded as argument but we need to remember to be cognizant of how context can illuminate or hide the intent/argument that's being regarded as fiction.