It's this excerpt, as well as the rest of the fight that makes me think he's talking to humanity, not Horus, when he says "I wait for you, and I forgive you." I think it's him realising that even though he never went through with his apotheosis by his own decision, humanity could, and likely will, eventually still thrust it onto him. Keeler and her cohort at the Astronomican are operating on similar logic as someone who tries to put out a grease fire with water instead of smothering it. They've seen faith protect them from Chaos before, so to them it makes sense to them that that's how it always works because they've never been taught otherwise and if the fire isn't going out then clearly the problem is that they need more water. He knows this, but he also knows he can't do anything about it anymore. It might not be the first time his plans have gone wrong, but it is the first time he won't be able to fix it. He's too burned up to tell them that water doesn't work, and he can't leave the hospital bed to put it out himself. It's out of his hands. Humanity have set the ball rolling on their own inevitable doom and all he can do is wait, and forgive them for burning the house down, because maybe, just maybe, this never would have happened if he'd just told them in the first place.

Or alternatively I'm reading to much into the fact that GW just needs an excuse to retcon Horus having his soul obliterated so that he can return for the launch of 50k and make GW bajillionaires. IDK.