On page 135 of Hand of Corruption it says that the C'tan consumed the souls of the Necrontyr, while actual GW publications have had C'tan consuming the energies of sentient beings and not souls.
-3rd edition codex, pg. 26: "A sacrifice of men, but the precious souls are not consumed."
-Pg. 25: "…the deliciously focused trickles of electromagnetism leaked by the mortal bodies of the Necrontyr about them awoke a hunger in the C'tan quite unlike the one they had sated among the raging torrents of stars."
-And: "Their discarded husks would be consumed and their cold, metal forms would be free to pursue their vengeance…"
-Pg. 60: "The C'tan are energy vampires who devour the unique essence of sentient creatures."
-5th edition codex, pg. 7: "…the C'tan swarmed about the biotransference sites, drinking in the torrent of cast-off life energy…"
-And: "As Szarekh watched the C'tan feast on the life essence of his people…"
-And: "…harvesting the life force of the defenders to feed their masters."
-Chapter Approved 2004, pg. 75: "…its primary function is to beam the harvested life energy of a world to the reaper-ships in orbit."
Nothing about consuming souls, and the word soul is used plenty, so it's not like the writers just wanted to come up with a fancier way of saying "soul".