MC speculated that it is the same world but in the future, since the spells work the same. Not to mention the repetition of the term 'my kin' as a condition on the reincarnation. And in case english isn't your main language, 'my kin' can be both species or family. He was a human that learned necromancy, so the 'kin' went for family. But I also suspect it could be a meaning lost in translation.