it never mentions that and just says you need a hero's crest... and it literally says if mai went it would cause a paradox. I'm pretty sure as a hero's daughter, mai has a hero's crest and the line on page 10 is meant to say "the hero's crest like one mai has" or just "like mai's" but as it's currently written it says mai doesn't have the crest, and anyone like her going through the gate would cause a paradox. it might be a scanlation but it's a very essential detail in this case.Imperial, the big tit elf that created the gate, exists in the past as her pre-evolution form. If she went to the past and met herself the paradox would start, so they sent the little kid back instead.