Fantasy is often characterized by a departure from the accepted rules by which individuals perceive the world around them; it represents that which is impossible (unexplained) and outside the parameters of our known, reality .... (Steve Bennett)
The resolution of "The Enormous Radio" is intentionally ambiguous [ambiguous: open to or having several possible meanings or interpretations (Dictionary.com)]. It may be that Irene has had a breakdown and is paranoid. It may be that the detrimental effect of intrusive troubled lives upon tranquil ones is paralyzing. It may reveal that under the surface, well ordered lives, like Irene and Jim's, are silently turbulent and, inwardly, like their neighbors in turmoil.
Identifying the climax, falling action, and resolution, which is also a surprise ending, may help sort out why this ending is in keeping with the structure of the fantasy genre, defined above. The climax--the moment when the conflict is resolved and the direction of the outcome is fixed--occurs when Jim says to Irene: "I’ll have that da---d radio fixed or taken away tomorrow." This is when the conflict is essentially over. Falling action follows until the surprise ending. For example, it's falling action when Jim says, ":Is everything all right?" during the broadcasting of a Spanish suite.
This is also part of the manifold irony of the story because the rest of the evening proves everything is not all right as he begins to criticize Irene's behavior: "I don’t like to see all my energies, all my youth, wasted in fur coats and radios and slipcovers and- ...." Then comes the surprise ending. Irene says, "Please Jim. ... Please. They'll hear us." Irene has stepped over to the other side of expectation. She has departed from the accepted rules of perception and behavior: She believes that now their lives are broadcast over the radio to other homes. This is how the ending subscribes to the style of fantasy fiction.