In that case he could have simply said nothing about it, or that it's been so long that he can't remember. That would at least leave the possibility that he's a magister more open. Don't get me wrong, when I first saw him I assumed he was a magister. But the game simply provides no solid evidence of this. It's basically just a matter of appearances with him.
If he was a magister and wanted to prove that his desire for peace was genuine, then he could have explained that without drawing ire. He could have said that he was one of the magisters who invaded the Golden City, that he realizes he committed many atrocities in doing so, and that now he wishes to make things right by doing all he can to stop the blight. He could even offer to provide information on the other magisters (who would be working against him) as well as the locations of the remaining old gods so they can be slain by non-Wardens before the darkspawn reach them.
Lastly, I think the fact that Utha runs down the steps to help him proves that he is not a magister, or at least one that can't body jump. Why would she try to defend him if he can't die? She would just run away and meet up with him later. And the Architect himself was pretty adamant about not dying. He fought with all his might and magic. He also wielded no power to influence Wardens or darkspawn. The only ones he seemed to be able to command were those who were willingly following him.
So even if he was a magister, he's not a powerful one, and in some playthroughs he's dead anyway.
Easy enough to resurrect, Utha may have been loyal and concerned, and I think there's an easy case for wondering if the hero would help someone who committed the ultimate sin by entering the city. In his shoes I'd keep that to myself... That'd be like saying "hey, you might be andrastian like most of people, I committed the ultimate sin against your god, let's be pals".
Not the sort of thing you'd share with anyone especially right before a battle.
Remember too that origins had the story of the magisters, but a lot of the lore like
Corypheus wasn't written yet. Corypheus was definitely based of the arhitect's design though, and we've never seen another normal darkspawn like him.
But hey, bring back the hero, give the architect a cameo if he's alive or if he resurrects somehow, and tell his story. Right now I get the sense bioware won't revisit him at all, and that sucks.