Enma mangic sword theory is still false, because Oden left two scars from both swords.
It can't be that Oden's cut was deeper either, because Kaido was bandaged up for days after, more than enought time to heal.
Seems Oden was able to nullify Kaido's healing ability, so that his wound had to heal at a normal pace, leaving a scar. Maybe this is a haki ability, to nullify DF perks like regen.