They can be killed as the C'tan picked each other off until there were only four of them remaining. So not invincible, but as close to it as possible.
thinking to it, if i should appear in the setting of 3d edition codex, i would have 4 C'tan with an army of slaves.
which i recall it's the only and true one, despite your nostalgia and your insistence to pretend that a question with a related picture of actual 42° millenium could instead refer to a previous, largely retconnected version. It has been fun for a while to play your game, but i'm gonna put it to rest.
Like said, it's all make-believe. People are free to play whatever edition they like. fixed that for you He who makes the post makes the rules! You don't interpret Warhammer rules based on the pictures of the models in the army book / codex, why should you do differently here? It has gone on for much longer than expected. It's probably best to move on.