I thought you were trying to answer why the story of creation could not have put after והגדת לבנך. To defeat the טענה of גזל for פסח דורות, it would have been sufficient to put it after והגדת לבנך. The only קרבן פסח that would not be covered by putting מעשה בראשית after והגדת לבנך would be פסח מצרים.
And anyway, the כנענים would not have had a טענה on פסח מצרים for another reason as well: even according to the Minchas Chinuch בדעת Tosafos that someone who doesn’t own קרקע is also exempt from קרבן פסח, surely that הלכה had not yet been given at the time of פסח מצרים, seeing as it was before מתן תורה. So how would the כנענים know this הלכה if it had not yet been given?