So if a users presses a button that says "Do X":
Then the app sends a request to a server to do X and based on the response looks up the text that shipped with the app saying "X completed successfully" which it displays using the system font, that is fine.
Then the app sends a request to a server to do X and the server sends back the text "X completed successfully <smiley-face-character>" and the app displays it using the system font, that is fine.
Then the app sends a request to a server to do X and based on the response looks up the text that shipped with the app saying "X completed successfully <smiley-face-character>" which it displays using the system font, that is against the rules.
That seems incredibly arbitrary and nonsensical if that is an accurate description of the rule.