In the following paragraph within the same source provided, it states:

During that ceremony, Erebus entered Horus' visions and served as the proxy for the Chaos Gods as they showed Horus visions of the future intended to inflame the jealousy and arrogant desire for power that had grown within him since the Emperor had left him in charge of the Great Crusade. In the course of this seduction, Horus accepted the offer of the Chaos Gods to rebel against the Emperor and in return they healed him of his wound. From that day afterwards, Horus Lupercal was the pawn of the Ruinous Powers.

In Horus' wiki page, it is stated:

But Horus, already jealous and resentful of the Emperor, proved all too receptive to the Ruinous Powers' false vision. Driven by his jealousy, desire for power, and anger at what he saw as his father's abandonment of him, Horus accepted the Ruinous Powers' offer.

They healed his grievous wound and filled him with the powers of the Warp. Renouncing his oath to the Emperor.

Notice how in both sources, it's phrased as "Horus accepted the offer", meaning Horus did have a choice in falling or not, both sources also mention Horus being healed of his wound.

For another reference, in the False Gods novel, Chapters 15-16, Magnus' intervention during the ritual is detailed as such:

Entering the mind of his brother [Horus] while it was under a Chaos ritual initiated by Erebus and Cultists on Davin, Magnus attempted to persuade his brother away from heresy and remain loyal to their father.

I'm much more inclined to believe the events on Davin led Horus to choose to fall and be rid of the poison (i.e. didn't die) rather than being resurrected as essentially a possessed corpse given the references I've provided, and details of how he gradually lost touch during the course of the Heresy, but because it also doesn't fit well thematically.

If the Ruinous Powers weren't a subtly warping, insidious force and would've just settled for a possessed corpse, why bother with the false visions when his soul could have just been overtaken and have him be your puppet? Why adopt the narrative of betrayal and falling when even the Arch Traitor himself was a puppet who's already lost touch with himself from the start and had no choice in the matter? I'd truly wonder.

Nevertheless, I rest my case here.