I eat açaí all the time (I love making 'sorbet' with the pulp and some frozen berries), but I haven't noticed any weight-loss effects. It is a very nutritious berry, with scads of phytonutrients and a high orac value.
As for Dr.Oz, I have a suspicion he promoted using açaí berries, NOT this particular supplement (I'd pay close attention to the wording on the site that claims he is promoting the supplement. It is probably worded in a way to make it seem like he's for the supplement, when he's actually for the berry itself).
I highly doubt it's just açaí that is the cause of the weight loss. It may be they are taking açaí the way it is often served in wherever it's from: mixed with guarana, which can have a metabolism-stimulating effect (I use the plain açaí, just berry pulp and some lecithin to keep the texture).
Losing that much weight in that short a time (unless, perhaps, one weights 600+ pounds to start with) is not healthy, safe or natural, IMO.