There is absolutely no reason why it wouldn't be Sephiroth. He looks exactly like him, with shorter hair and a gay costume. On top of that, the only characters that we have seen in any released screens are Cloud and two dudes that look like Sephiroth. One would logically assume that the two guys that look alarmingly similar are, in fact, the same person.

Even if it is a clone of Sephiroth, that still technically makes the character Sephiroth to some extent. Stop nitpicking over a slight height discrepancy or a new sword. Whether you like it or not, the goth queer is Sephiroth, plain and simple.