Scorsese is attacking Marvel and superheroes as a genre. Gaiman sees Scorsese as a natural ally since he has never been on board with the idea of superheroes being the be all and end all. Don’t see what’s to misunderstand there.
I think because superhero characters are extreme. They don’t die, don’t grow old, rarely have sex and morality wise you can’t have them be too compromised or flawed. Most movies which are considered great deal with characters who aren’t bound by that. Now of course there are exceptions like Logan but that’s not really representative of the genre.
They can’t really speak about big stuff like history. Captain America First Avenger will never be better than Casablanca, saving private Ryan, Best Years of Our Lives in terms of saying stuff about world war 2.
The Godfather and Chinatown were movies that were about criminals who get away, heroes who become corrupt and so on. You will never have a superhero movie that goes all the way there. If they do it will be a cliffhanger and have a stinger and next movie you will see it undone as happened with Thanos.