I didn't say they wouldn't lie, I said they probably didn't because what would it achieve, yes Daemons lie but they don't always lie. You can't just dismiss evewrything and anything that Daemons say throughout the HH novels just because they are daemons and are known to lie. I mean this is a logical fallacy, if you read that you would come to the same conclusion as there is no obvious reason why they'd lie but because we are having an argument, instead of taking it at face value, you look for something that will dispute the evidence. Because Its surprising you's didn't know that, its widely known and everyone else takes it as being true.
We do know Magnus was the first choose of all four powers, otherwise they wouldn't have mentioned Horus. If it was Tzeentch then to compare Magnus to Horus, Horus would have to have only been chosen by Tzeentch.
What would it achieve? It's a Daemon of Tzeentch, lying is like breathing to them. After all, this is a God who's right hand and future seer (Kairos) gives contradictory, deliberately false yet plausible information in addition to his prophecy. And these prophecies are only granted to the greatest Champions who have served Tzeentch well. Yet he s with them. Alternatively? The lie helps screw with Magnus's head, as iirc he's going through some pretty 'mind y' stuff by that point of ATS.
When a Daemon makes a weird claim like 'The Gods chose Magnus first, but he was too powerful, so they settled for Horus, yet Magnus ultimately falls to Tzeentch alone', a claim which appears nowhere else in the fluff that I'm aware of, it's not unreasonable to doubt it. Especially because it seems pretty outlandish. If Magnus was 'too strong' to be the arch traitor, why did the other 3 let Tzeencth claim him? After all, isn't one of Abby's 'special things' that he's such a big deal the Gods trip over each other to try to get him to swear for 1 of them, because that'd give them a major leg up vs the other 3? If Magnus was all that, it seems plenty weird that the other 3 Gods, having dismissed him from consideration for 'Big Bad' because he was so powerful, to then sod off and let Tzeentch have Magnus all to himself. So, overall 'the Daemon was lying' seems a more plausible explanation for the events presented.