This goes into GM's state of mind. If it was their preferred outcome how would this look any different in play? That's right, it wouldn't.
This is odd. I mean, for this to be Force, the GM would have had to have planned it and wanted this outcome -- a bird appears and threatens to attack. Note, it doesn't attack, and the game doesn't shift into combat mode because there is no such combat swoosh in Story Now games -- play continues using the same framework. The bird appears adds a new threat, a new source of hard moves to the GM's repertoire, it doesn't create a set outcome. The bird can appear, the PCs can take an action to deal with it, and the bird can be gone just like that! Or, they ignore it but nail successes and the bird just wanders around without anything to do. You seem to be viewing the introduction of the flying monster as an "aha, now you'll be forced into my combat encounter!" That's just... not what's happening.
You frame the situation, you describe the area. You can easily make it seem natural. And if for some reason not in that exact moment, then in the next. Hell, by creating correct atmosphere you might even get the players to suspect the presence of infernal influences and make them think that introducing the demons in the narrative was their idea! (That's trickier though.) But if in fantasy milieu you want to work demons in the narrative in way that seems organic, you absolutely can!
Again, you're assuming the framing is free on the part of the GM. The cliff being climbed isn't free to frame, it has to flow from prior play. But, let's ignore this. What you're trying to call Force here is the point in the game where the players have chosen to engage in an adventure that called for the framing of a climbing obstacle, and the players chose how to engage that obstacle, and they made their move, which, on a success, overcomes the obstacle. But, you want to call it Force when they players fail and the system say is that the GM gets to introduce a consequence to make the PC's lives harder, and they choose to introduce a flying monster? I mean, that's a pretty long chain of things that have to happen just so you can say it's Force for the GM, when given permission by the game, introduces a complication, not an outcome but a complication, to a scene and still has no control over the outcome reached?
Okay. I mean, I disagree and think that's an incredibly strained argument, but okay. And I say this because when I was in your shoes, and was facing this exact argument, I said the same things you're saying. Like, pretty damn close. My bit was "well, if the GM wants the game to be about demons, he can just introduce demons at every chance he gets and that'll do it." I'm slightly embarrassed by myself now that I've actually got experience under my belt both playing and running and recognize that this is just not possible without being blindingly obvious in game and also very much against the rules. I mean, sure, we can talk about degenerate play examples, but I don't think that me suggesting how you play and only using people clearly ignoring the game to do the things is going to go over well with you.