I think they meant if you'd used that alternative clue, the answer would be in keeping with your theme? But then you'd want to make the top center clue a theme answer and it'd be a whole thing...
Add a space: "E as in echo." (It's not a great entry, sort of "green paint"-ish, in the sense that you could replace that first letter with anything and claim it's a valid entry, e.g., "B as in Bob," "A as in awesome," etc.)