The main question is, is it an apparition of a conscious Sephiroth reaching through the Lifestream and the Jenova cells inside of Cloud, or is it simply an illusion conjured up by Cloud's mind as a side effect of his PTSD triggered by the destruction he sees in the streets of Midgar and amplified by the Jenova cells within? In the context of the original storyline, the latter makes more sense to me since Sephiroth only started to mess with and control Cloud after he gets in contact with Jenova in the Shinra HQ. To my understanding anyways.
However, the lines do seem like Sephiroth is starting to manipulate him into delivering him what he wants, the Black Materia. On the other hand, I don't think this happens this early in the game (yes, I still believe this happens after Jessie gives Cloud the Materia and before he meets Aerith and it's probably the reason why she's asking him if he's OK, with him being still confused from that Sephiroth illusion) and since Cloud does posses suppressed memories in his mind which can surface at any time just in different forms, such overlaps like Zack's words with Sephiroth's illusion has a good chance of happening. Not to say those are Zack's words from the rewritten Remake dialog, but it still is a possibility.