See Sephiroth you and I read that line differently.
I see it as him saying stuff like the keyblade war is an important event to the KH story, which it is, but we don't need to experience the keyblade war to understand it. We've got the ending and most of the details to the war all we were missing was the spark that aggravated and changed things, and that's what BC gave us. BC doesn't let us experience the events like that war, but the experience in unnecessary to understanding the story, it's just fun to be a part of it. That's why I compare it to Rogue One, we never needed to see that tale but getting the plans for the Death Star was still of vital importance to the Star Wars story as a whole. BC works as a stand alone tale, you can watch BC and you get pretty much everything important. Like how the only thing the Nightmare Chirithy Dream Eater really added to the tale was to aggravate trust between the foretellers/union. Was there was story to the Nightmare Chirithy? Yes and that is what Browser Chi's story was ultimately about, solving the mystery of the nightmare chirithy that helped cause and spread distrust. But BC not personally addressing and resolving that plot point is irrelevant to understanding the greater story being told.
We already know how the keyblade war ends, we already knew going on that it became a fight over kingdom hearts and the X-blade, we already know the keyblades were forged to help obtain kingdom hearts, we already know the fighting breaks out over greed over light, etc...
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BC gives us the rest we need to know by showing us the leaders of the faceless masses that instigated all this. We know who forged the first keyblades and while the foretellers don't know what they are for, we as players have the hindsight of the future to tell us that. We know the light spoken of in legend was actually a force of light called Lux. We know the forces that fought the keyblade wars are unions lead by these foretellers. We know Gula is going to summon kingdom hearts and from there it's not really a secret how things will spiral off from there. We know Luxu is the only one who will live past the keyblade war. And we know now why and how Master Xehanort got his hands on that keyblade, KH3 will explain the full ramifications of that in the future.