That was indeed what the Sparkles had told Spike, which was kind of a half truth if you squinted hard enough. Explaining in great detail the method Sunstone used to depart from the mortal realm was deemed a bit too much for a dragon of his age. Analysing the sort of mental strain and depressive thoughts that would push a pony to nullify their own life was not something Twilight wished to explore with someone who had yet to enter puberty. Spike had every right to enjoy his childhood before being exposed to a sad reality that was too often ignored and/or brushed away in modern Equestria.