I'll be the first to call the Horus Heresy novel series a steaming pile of gak, especially anything involving Perpetuals,
but to be fair Perpetuals can be killed.
In
Unremembered Empire they reckon a a chunk of Fulgrite created by the Emperor's psychic lightning striking sand is enough to kill Vulkan if wielded by a Primarch.
Now that would be only the tiniest fraction of the Emperor's power, so imagine what Horus could to do after being granted tremendous power by the Gods.
Alternatively as in
The Lost and the Damned, the Emperor over drew on the power of the warp and in doing so undid his immortality.
gungo wrote:I have a feeling the golden throne is just what the emporer needed to become the star child
Going by the Star Child fluff the Emperor is already the Star Child, and he needs to be reborn as the New Man once more to save humanity.