Oh 100%, just watching Back Cover would give you the bits of Ephemer and Luxu that you needed, but it's their lived experience with the series and they felt exhausted at the end of III, feeling like they didn't want to keep up with a story in a mobile game in order to move forward with the console series which is fair, since UX and Dark Road did continue after III released and introduced the Darknesses and all that. Plus there's Strelitzia who was mobile game only so far.
The same discussion will be done with Missing-Link and IV.
"I don't want to play three mobile games in order to understand what's happening in IV"
"You don't need to play any mobile game, everything is introduced in a way to the audience through Sora who also doesn't know."
"What's the deal with this?"
"Well in the mobile game...."
And repeat ad nauseam.
But this is the life we chose, fellow Kingdom Hearts fans