Looks like forearm to me.
I don’t think wrist motion is compatible with flexed wrist. I can’t make it happen at all, even super slow.
don’t gypsy jazzers use a wrist motion with a flex’d wrist? (maybe not 100% wrist, but mostly…)
No, flexed form uses forearm and wrist. Elbow motion is not forearm motion, although elbow is often used in conjunction with forearm and wrist for strumming.
Now it may sound strange coming from one of the CTC team but I think you are overthinking this strumming thing a little 
I don’t think the great strummers out there micromanage the joints they are using. There’s actually no reason to impose yourself a “wrist only” restriction on this. The variables worth working on are questions like: is my pick grip comfortable? Is the pick flowing smoothly both on upstrokes and downstrokes? Is my timing good etc.
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I’m not even sure the “smooth” thing is even a consideration for this most of the time. Strumming in general isn’t really a smooth thing.
I also don’t think most of the great single note pickers micromanage their joints movements either.
I may have used the wrong term! By “smooth” I mean that both the up and downstrokes shouldn’t feel excessive resistance from the strings (i.e. you want to avoid the usual garage spikes problem).
Oh I got you, and I’m not sure garage spikes is really much of an issue for this in the same way it is for fast lead string changes or other rhythm styles. At least not typically in funk comping which is fairly mechanically aggressive anyway and largely directional by nature anyway. It just needs to be smooth in the rhythmically accurate sense.
You might be more concerned with that in heavy metal playing, where you have a lot of faster one or two string riffage combined with palm muted chords.