This is something I like to point to when people say that KH3 expects you to know Union Cross -- it doesn't. At all. If you know Union Cross, you're like "OH SHIT IT'S EPHEMER" but KH3 is clearly written with the intent of the Union Cross related stuff being mysterious to people who don't know Union Cross. Even the Luxu/Foretellers stuff is completely valid as "Who are these guys and how do they know Xigbar?"
"The mobile game is important!" is true, but not in a "if you haven't played it you're lost and they're never going to explain it!" way, it's important in a "If you played the mobile game you know where stuff is headed and get a different, but still surprising, reaction to stuff" way.
It's actually kinda cool that they're writing it in a way that works both to newcomers and megafans.
EDIT: Also, a reminder that the journal is written from Jiminy Cricket's perspective. The game cheats this with "The Boy in White" and "The Boy in Black", but for the most part, it works within the rule that Jiminy has to know for it to be in the journal.