He's not the Ardyn we know. This place is capable, just like InterGal, of pulling people from different times and dimensions. It's quite interesting, but also fucked up. Basically he's an Ardyn from another time and place.

[Nari knows some things about Ardyn that CT doesn't from being a survivor.]

It's not erased memories. This one literally doesn't know us. Just like the Mira here is from another dimension from the Mira from my world.