Horus hid his abilities until the end.

"Go,' says Sanguinius. 'I need no more lessons.'
'No, one more,' says Ferrus. 'He hid in the past.'
'Horus?'
Ferrus Manus nods. 'That's how he tricked you. That's how he tricked our father. He could not risk our father learning the extent of his power, or reading his thoughts, or comprehending the nature of the trap he had set, so, by will and warp, Horus put himself into his own past, into his memories, so deeply and so completely that, for a time, even he did not know where he was or what he was doing.'
'Is that possible?' asks Sanguinius.
'For him, yes. A tactical masterstroke, but then he is the Warmaster. There was no mind for our father to read, no present thoughts that could be betrayed. Horus gave himself to madness and fitful recollection so that he could give nothing of his plan away. And his plan was known only to him, of course.'
'And thus we were deceived, and came here,' murmurs Sanguinius.
'Yes,' says Ferrus sadly. 'And the need for such insane cunning is now passed. You are here. So his mind is restoring, rapidly. He is becoming himself again, quickly, himself and more besides, fully willed and cogent, present and all-powerful in this moment. I tell you this, brother, because it takes huge effort, even for one such as him, to reconstruct a mind so thoroughly dismantled. He is not quite there yet.'
'So he is weak?'
'In body? No. But in thought, perhaps still, a little. Traces of the self-inflicted dementia may persist. At the very least, he may be confused or not self-assured.'
'So, a flaw I can exploit?'
'Perhaps, while it lasts. Which won't be long. And even weak, he is–'
'I understand.'"

-The End and The Death: Volume Two

The next book extract is explicitly from Horus' perspective on hiding his abilities.

"Your father is just a man, after all. He is brimming with fury, and burning with power. Such power. He is shining in your mindsight like a star. You had Him encased in a stifling, muffling, sense-depriving shell of pure, black immaterial force, but He has cracked that open, and now He burns a path towards you.The Vengeful Spirit creaks and shakes at the fury of His approach. He is so strong, so powerful, any being in the galaxy would shriek in eternal dread and hide in the depths of hell rather than face Him. But there is nothing in the galaxy like you any more. You no longer have to hide. You no longer have to conceal your power behind veils of secrecy and deceit. If He'd had an inkling of how strong you'd become, He would never have dared come to challenge you. So you wrapped all of yourself in un-when, in skeins of timelessness... your presence, your thoughts, your soul, your power."

-The End and The Death: Volume Two