This is probably one of the most profound scenes in One Piece and it never gets discussed when Luffy's characterization is the topic.
It's not that Luffy is ignorant of the world around him. It's not that he doesn't have the capacity to philosophically engage with the surrounding circumstances.
His experience with Gray terminal and Sabo's death made him fully understand what the outside world stands for in the One Piece story. He's come to terms with the limitations the WG puts on human dreams and pursues that meaning to its fullest. That's why if the world changes his ideals it would be contradictory. He must remain steadfast and true to his core beliefs while the world bends towards his philosophy if the series is to remain thematically consistent.
For example, Gon from HxH never had the experiences Luffy had as a child, hence he's still naive and never came to terms with the world before his adventure. That's why his character breaking in Chimera Ant (and the contrast with Killua who knew what the world was about) was brilliant because he never understood the depth of the world's darkness.
Luffy already knew before he set out that if you want to be free you'll put your life on the line. The scenes where he ignores the world news, or sleeps during important flashbacks are perfect character moments. There's nothing that someone can tell him about the world that he doesn't already know. There's nothing about the world that can change his ideals. He doesn't ignore because he doesn't have empathy. He ignores because he already reached an unshakable conclusion.
In chapter 10 when Buggy ordered Nami to kill him, he already told us what a pirate means in One Piece, he doesn't need to change.
TL:dR
Luffy is "flat" as in he doesn't change his ideals. But that's exactly why he's perfect for this narrative.