Now I'm more confused than ever.
Why would Mike Laidlaw think that it was "obvious and widely known"?
What percentage of the player base does Bioware think actually reads the tie-in material?
I think it's likely they assumed those that read the tie-in material would spread the word. After every DA related piece of news, from new games to novels to dev tweets, these forums alone blow up with tons of posts and threads filled with people saying "NO WAI!" "YES WAI!" I guess they figured it was the same with Alistair's parentage.
I'm not bothered that they thought this was common knowledge as much as they assumed people took for granted that it was canon despite the ambiguous ending. BioWare plays fast and loose with canon all the time. The end credits to Origins and Awakening are "just rumors," often ignored or contradicted in future games, companions who can be killed in some playthroughs turn up no matter what in future games (like Leliana and Anders), and any vague information can mean anything goes (like Maric's definite death at sea 5 years ago turning out to mean he's still alive).
Since The Calling didn't come out and say "And they named Fiona's and Maric's baby Alistair," many people were afraid to just assume it was him. What if Gaider kept it vague on purpose so they could introduce another royal bastard in future books, comics, or games? What if X, Y, and Z? Then Laidlaw says, "Sorry, thought it was common knowledge."
UGH! We can't win!