Well, since we already know that the Magi boon did not happen in the novels, and current information and statements indicate it will not have happened in DA:I, I'd say developer intent is pretty important. People don't have to like it, but unless their idea of 'enjoyable' is being annoyed every time the writers disregard an option to suit the story, I suggest they accept it.
I'm curious about whether or not you realize that the reason there is no magi boon in the novels is because the Warden isn't a mage, the Warden is a Dailsh, who did the ultimate sacrifice. That means the novels cannot disprove the idea that the magi boon did happen in world states which have a mage asking for it.
If DA had ended, then I'd agree that author intent past that point would have subjective value. If they aren't making anything else to contradict the source, then their intent is only as good as their execution. However, in this case the writers are continuing to create the world. As such, they determine what did and didn't happen on a per-entry basis and they hold ultimate authority. And they have explicitly stated that if something gets in the way of the story they want to tell, it gets disregarded.
When they discuss something in an interview/tweet/forum post, they aren't doing it to sneak some important bit of information past you. They are using the available and acceptable methods of communication to set expectations for the next installment. The developers are telling us now in the hopes that when people finally do have the finished product to play, they won't be surprised when their pet choice isn't represented. If you don't see it or don't hear about it, it's not because they haven't made the effort to tell you. And if you disregard the information because you don't like what they're telling you, that's your problem, not theirs.
Yes, developers are using various outlets to set expectations for the next installment. But, as said, until the next installment actually comes out, and verifies what these developers are saying, the words that comment on a prior game's events are just interpretations of those prior games. After all, if Edmonton were to be the subject of a nuclear explosion before October, it's debatable a game that confirms such words would ever come to pass.
Essentially, the only thing one can go by is what is actually in the game. If it's not in the game at all, you can use developers statements to make your case about the subject, but no one is obligated to accept that as canon, because it's not actually in any of the games, and thus not officially canon. I, personally, agree with Gaider that the Chantry would have stepped in and stopped the Ferelden circle from being autonomous(or tried to stop them). But, based on the games, no such action is mentioned. Only that the mages were declared free by King Alistair. And as such, claiming that such an event is canon is less than genuine, because until those events are stated to have occurred by Dragon Age:Inquisition(and not just by a developer), it's not yet canon...