From what I read other places, it meant that if it was non-variable and shipped with the app (UI, app data, etc) then it was "media" and not ok. If it was user supplied (like a text message), externally loaded (web page), etc... then it was "content" and ok.
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.
Another link was posted suggesting that Apple's reversed this stance. If not for that, would Apple have been making Slack get rid of the party popper because the user didn't type it? It sounds like yes.
Apple has thought of Earth as their personal playground for a long time; arrogance runs deep in their company culture. Steve could get away with it because he was arrogant and right most of the time. Today, ancient & obsolete arrogance is Apple’s biggest flaw, and is causing them to drift more and more out of touch with reality.
Even in really weird ways, like: to install any software on your Apple phone you had to use Apple's music player. (Is that right? I never owned one, and it sounds daft, but plausible for Apple.)
Text and video messaging was low-hanging fruit, thanks to XMPP etc not taking off. With that network effect in place, it's a short leap towards a subtly-incompatible text format, so buying Apple's stuff becomes the path of slightly less resistance.
I don't know whether this is an aggressive strategy; or whether they care only about their customers' satisfaction and not at all about interoperability.
Uh...no? Not sure where you got that from.
A now-deleted tweet by a different developer facing the same issue stated that Apple apparently clarified by saying that using emoji as "media" was forbidden, but "content" was okay. Whatever that means.
Between this and the gun/water pistol thing, it really seems like Apple thinks of Unicode as their personal playground, and simply do not think through the network effects of their policies at all.