Gaider made mistakes. He's human, happens to the best of us.
Those mistakes, however, resulted in consequences.
The books are part of the lore, yes. No one's denying or disputing that. But the games are part of the lore, as well. And what some of us(myself and EmperorSahlertz) are asserting is that as far as we are concerned, the games are a bigger, more important part of the lore than the books.
I'd also like to point out that Fiona's child is never named in The Calling. If he wanted to be clear about it he could have simply had Maric as and Fiona answer, "Alistair. His name is Alistair.", or some such.
There's this concept spawned by an essay titled Death of the Author, that argues against including a creator's intentions into one's readings of the work, rather just taking the text as it is. Under this school of thought, David Gaider's thoughts and interpretations on Dragon Age are no more valid than that of any other fan of the work, until they get published or included in the game.
There are parts of Death of the Author I agree with and parts I don't agree with, but in this case I'm invoking it; as far as I am concerned, Gaider's intentions that haven't yet made it into dragon age canon aren't my concern.