What you see (wrist flick) is different, yes - but it actually is very helpful (overall) in terms of hand and wrist flexibility for djembe. If you can already create a slap sound, it's just a matter of adjusting hand position to do it properly on djembe... however, if one can play a slap, one can play a slap, imo.
(Though I think my slap comes mainly from frame drum playing, where a lot of different slaps are used.)
I don't use "wrist flicks" on djembe, except for when I want to play a slap a little differently, or else during times when I'm playing the drum with darbouka technique (for myself; just having fun).
Also, I think the term "wrist flick" is a bit deceptive - some people don't flick the stationary hand at all; they lift it and bring it down on the rim of the darbouka. That's probably not an optimal way to play, but some of the older Middle Eastern players were taught to play in that way and have passed it on. (I've mostly seen it with a few folks from Lebanon and Americans who were taught by them.)
The Gnawa have their own way of playing frame drums, too... but I don't think it's confined to them. I've seen a bunch of "unusual" (to most Westerners) techniques used by other N. Africans - people who also play in the "classical" Arabic style. The blending of cultures in Morocco, Algeria and Tunisia has produced some pretty remarkable things!
(btw, i believe we've been around the block on this topic a few times... maybe a video would clarify what I'm trying to describe, but I don't have a decent camera, so you'll just have to take my word for it. Cool?