I always found that explanation of the "TWD" universe a bit of a cop out.
A lot of fiction actually strives to not be too self-referencing. In many creative universes, Hollywood is basically invisible.
It's really no different from people in superhero movies not knowing that superheroes exist, except the one(s) they encounter which surprises them.
The typical people of fictional stories don't really know what we about when it comes to lore and cultural media references, unless it is important for the plot.
Most stories that acknowledge the different memes, lore, and references, are usually light-hearted. It's often paramount to breaking the fourth wall. The characters in a Hollywood product are referencing Hollywood productions! GASP. It actually does more to harm the suspension of disbelief than you might realize, thus why it is rarely employed in anything attempting to be serious.
Thus, Zombieland can get away with it, but not The Walking Dead. It just wouldn't work.
Movies like I Am Legend didn't reference zombies either.