The point being, nothing can be said to have not happened. We just are not focusing on it. If a writer really likes something from the seventies they can tie it in, and if it really takes ahold as an idea it could easily become important enough to end up one of the key points for the character, and so end up in the next handbook.
I know this isn't a popular thing to do based on the way fans think, but imagine you were a writer. Let's say you know everything about Spider-Man but Marvel ask you to pitch a Dr Strange story. You are not going to turn that down, you could do a great job and end up on Spider-Man but you don't know everything about Dr Stange. You make a pitch and the editor likes it but he points out some inconsistencies, you acknowledge you are not an expert so he hands you the three key storylines he thinks you should need. That should be enough, the core idea is right there and the editor is ok with a bit of interpretation. You are free and able to write the way you want, and not feel constrained by something that happened in the seventies you don't know about. You have time to write your first arc and in the meantime you can do some more background reading.