It's also strange that an Alistair who is made king never mentions the fake Calling.
The Calling is a Grey Warden secret told on a need to know basis, and King Alistair isn't involved with the Inquisitor by the time the Inquisitor needs to know even as much as they end up knowing. (Or at least he's not involved in the mission that requires that the Inquisitor know it.) Besides, the secret that the king has decent evidence he's going to freaking die soon isn't the sort of thing you spread carelessly.
And this is assuming Alistair even heard the fake Calling, since we don't know how the spell that created it was targeted: if it targeted every being with a Warden body in Southern Thedas, he hears it, but if for example the Nightmare demon has to specifically choose to target every single person who's supposed to hear it then you wouldn't necessarily expect Alistair to hear it except as part of a plot conducted offscreen. Now, an offscreen plot to gaslight King Alistair would be a clever idea on Corypheus's part, because the childless King Of Ferelden committing suicide to avoid becoming a ghoul or desperately fleeing to Soldier's Peak to try to get his body under control or doing some other extreme reaction that precludes actively kinging around would be all that country needs during the events of Inquisition. (Particularly if he's ruling alone or with a wife who's also off researching the Calling.) Heck, even if Alistair keeps his head for the most part, he won't be at the top of his game dealing with a threat like this, and a distracted king is a win for anyone who wants to conquer his kingdom. But we don't know that any of that occured to Corypheus, or even that he knows that Alistair is physically a Warden unless Alistair is serving as one.