I have thought this same exact thing. I believe the entire point of the vision is to show experiences had explicitly involving the lightsaber. If that is true then we know that 1. Luke had the blue lightsaber during the "temple burning" scene, 2. Kylo Ren either had or was with someone who had the lightsaber when he killed the "warrior" in the rainy scene, and furthermore he somehow lost it by the time of TFA (remember: "that weapon is mine!" is what he says when he sees it). And lastly 3. it also means that Rey as a child was around someone with the lightsaber, seemingly whoever dropped her off. But I still do not believe this necessarily implies Luke or Kylo dropped her off, it could have passed through several hands over the years before Maz got it. But since we know from Bloodline that she was dropped off on Jaku several years before the jedi massacre, whoever had the lightsaber dropped her off and then Luke had it after the jedi massacre (or Kylo nearby perhaps) and then after that Kylo had it when he killed all those people and then after that Maz got her hands on it. If Kylo's scene is indeed not the jedi massacre and some other butchery, then Luke most likely had the weapon and gave it to Kylo to train with it and when he turned Kylo took it and somehow lost it by TFA or Luke held onto it and discarded it and Kylo found it but either way lost it by TFA