I remember seeing a theory that Sandman.. this Sandman, is a different one than who we saw before. Valentine used his stand to bring an alternate version of Sandman in. The proof of it is some small things.. but the one I remember now is him saying he has the name Soundman, his tribe calls him that. In the beginning of the comic, his tribe, his sister, calls him Sandman. Yeah sure, this can be explained with Araki forgot, but the theory is much cooler