You an buy audio book from Audible, you get to access it on the Audible app on any platform it's available.
You buy an audio book from Apple, you get to access it on the iTunes app (I think) on any platform it's available.
You subscribe to Disney+, you get to access it on the Disney+ app on any platform it's available.
You obviously don't buy an audio book from Apple and get to access it on the Audible app on any platform it's available. You yourself said this as if you didn't have a problem with it.
And yet it's exactly what Epic want as well... buy Fortnite DLC from Apple and get to access it on the Play Store app from Google, Galaxy Store from Samsung, Steam from Valve, and other unrelated platforms.