That's NOT a retcon. That was in the actual story itself. And that's the official explanation.
Goblin wanted to torture and torment Peter by making him believe he could have saved Gwen, only to rub it in his face that he never had a chance. That's even more cruel.
It's NOT a lot. Only Slott's Clone Conspiracy runs with that...and that's even more ridiculous anyway.
According to Conway it was a total accident.
Tom DeFalco: In your mind, was Gwen still alive until her neck snapped?
Gerry Conway: "Could be! Honestly, I don't know — I'm not sure why I added that sound effect, or what I meant to accomplish; as I say, it was the result of a subconscious decision. Consciously, I've always thought that she was already dead when Spider-Man caught her. But if that's true, why did I put the 'SNAP' in? What was the purpose of it? Spider-Man couldn't hear it. It was strictly for the audience. What was I trying to say? That 'SNAP' came from a pure artistic impulse. It was not calculated or part of a master plan to mess with the readers' heads...That said, I'd sure like to believe she was already dead."
-- Comic-Creators on Spider-Man, Titan Books, 2004 Edition, Page 47-48.
To me the idea that Gwen got her web snapped by Peter runs counter to the story and overall direction of the arc. If Gwen's death is a result of Spider-Man's incompetence then that means Peter and many writers after him are wrong to blame the Green Goblin for her death...and the entire rage and emotion in ASM#122 is unearned and unmerited...which is obviously not what Conway believes and not what anyone believes. The story continues to treat Goblin as Gwen's murderer. Green Goblin killed Gwen, Gwen died because Peter accidentally killed her. Both of these things cannot be true.
As such to me "the fall killed her" offered by the story is the best explanation and in real-world physics it checks out.
Likewise, there's the problem that Peter repeatedly yanked people mid-fall multiple times before and after that story and all of them survived.
Gwen Snap Spidey 1.jpg
Gwen Snap Spidey 2 - ASM #232.jpg
Gwen Snap Spidey 3 - ASM #362.jpg
The first one is from the Lee-Romita era (you can see the distinct 4-Rectangle Panel that JRSr used, the second one is from ASM #232 by Stern/JRJR, and the third is from the Michelinie era, ASM#362, by Bagley). The latter one has Peter using his webs to save a baby...and I should think that if a tiny child's soft neck survived that, then Gwen's neck couldn't have snapped just because Peter used his webs. There's actually more. You have an instance of this in BACK IN BLACK but there's a limit of 3 images per upload.