There is no doubt for those who can read instead of trusting John Brittenham.Hojo never lies with his informations. He says Sephiroth uses JENOVA when he arrives at the Knowlespole. JENOVA is an alien which can think like a human bein, but wants to follow its instincts mainly: Inject other lifeforms its viral cells, absorb life & reunite.Sephiroth, who is also the real Sephiroth during the journey and Advent Children, since it is his original soul and conciousness has those powers because Lucrecia got the cells. Not Sephiroth himself. She says, the JENOVA inside her didn't let her die. Sephiroth was created by the JENOVA cells fusin with Lucrecia's egg and Hojo's sperm right at the beginning. He then became even stronger in the lifestream - only strong-minded beings like Sephiroth or Zack resist it - absorbed life power and knowledge/memories (which are the source of power in Final Fantasy VII) and learned to use his alien powers.