its said at the end of hardmode that its canon; well, the event in em at least
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I mean how would Eve eat Toriel twice? lol. Unless she escaped the first time or something and just forgave Eve I guess? Personally I think Hard Mode is probably only canon so much as Eve falling into The Ruins, meeting Toriel and arriving at her house. Not so much the eating Toriel in her chair part or the two extra routes. So Hard Mode is probably only canon on a very loose level.