To give an example:
Let's say I want to write a story. I decide to write a light-hearted, fluff fic; the premise being Kerry, Iri, Shirou and Illya spending some fun happy times watching the cherry blossoms bloom. Maybe include some in-jokes based on the source material, perhaps a few cameos by other characters here and there.
Is it breaking canon? Yes, of course it is. Does it matter? Well, if the story is good enough...no. Not really. Do I need to explain why Kerry and Iri are alive, and are having happy times despite what happens in the source material...no, not really.
Because that is not fundamental to what the fic is. It's just a small fluff story. So as long as I state it's a what-if/AU story, and make my best effort to write everything properly, people are very likely to overlook the fact that some of the characters in it are supposed to be dead, or that the situation in itself is not possible in the actual source material. I could include a small aside hand-waving it or something, but it doesn't need the explanation. People already enjoy the story enough to give it a pass.
Your story, on the other hand...needs the explanation.
Because due to what your story is, and how it's supposed to go, it needs an actual explanation to why it happens. It follows an established setting, so it needs to be explained within the setting, and the explanation needs to make sense so as to not break SoD. Otherwise, you have a plothole or an asspull.
Even if the setting is explicitly AU, you'd still need the explanation, because it's fundamental to the working of your story (and if it isn't, why break so much just to have them revived in the first place?).