I wanted to use ogres (ogres are cool!), which meant that the Qunari could not be
totally removed from the rest of the world, since you need to have Qunari to get ogres. When I started writing this book, I had not yet read the World of Thedas, so I didn't know about the darkspawn-slaughtered colony in the Korcari Wilds. That would have been a shortcut to the entire thing.
But, since I wasn't aware of that then, my reasoning went: a world that has high-speed aerial travel is a "smaller" world than one that doesn't, and a Grey Warden who just happens to fly overhead and spot one of those guys is probably going to be all "whoa, what is THAT?" and drop down to investigate. Therefore it would not be completely unreasonable to think that isolated contact might have been made once or twice, by that or other means, and that at least some Grey Warden scouts might be at least broadly aware of what Qunari are (if only to the limited extent of "hey, these guys are not darkspawn, we don't need to freak out about them even though they look kinda weird"). But, of course, that's not their primary focus, and it's also not hard to imagine that the few Wardens who maybe hypothetically did know about Qunari got wiped during the Fourth Blight, and then there are no more griffons so the possibility of accidental re-contact goes way down, and we get back to a world-state where nobody remembers what Qunari are until they invade.
However, at the same time, I didn't want to portray Qunari as being so common or widely known that one would actually make an on-screen appearance in Last Flight, because then there would clearly be a canon conflict. To split the difference, they only got a one-off mention in a throwaway line. I figured that would cover all my bases as to who knew what when and how there could be ogres in the Fourth Blight, as the readers who cared enough to worry about that kind of stuff would be able to make the connection, and everybody else would just skip past it.
But obviously I'm not actually all that confident that it worked, which is why I wrote this whole long spoiler explanation about it.