Canon said the warrior from D1 beat Diablo and shoved the stone in his head. Blizzard simply expanded upon what happened, as a nicety (I think), for those people who used sorcs and rogues. (They become Blood Raven and The false Summoner in D2.)
They made the warrior from D1, who was clearly meant to be random guy #1337, Aiden to connect the story line from D1 to D2 to D3. D1 left the ending open for a sequel but the "adventurer" was not meant to be someone of importance. Blizzard later added he's someone important with freshly thought of expanded lore. That's why peopel say retconning even though it's not, but it REALLY feels like it.