I actually vaguely cited MSB in my example in that thread.
But yes, anything can be adult. You can tell an adult story and still have it be rated G. Younger writers tend to be under the impression that for a story to be "adult," then it needs to have drugs, sex, violence, and other "adult" themes. While, yes, those can be used, they shouldn't be the driving force of your story. Your story should still be able to stand on its own with those themes removed.
Do you have an interesting character? Do they have an interesting story to tell? Do we want to follow that character through their story? What makes that character interesting?
I'm sorry, but drug addiction does not make a character interesting. Drug addiction is a glaring weakness. It is something that needs to be overcome, and if the character's story does not allow them to overcome that weakness, then why should we care? That character has not grown or changed in any way.