No, it is not. I am going to be blunt with everyone.
Do people really think if this card could just insta-hack your system, that they would be this careless with it? Seriously if this card was that damaging they would have clear tabs on who's using it, what 3DS it's in, was it returned, who returned it, ect ect. They would never be this careless if the card was that damaging to the system. In reality if this really did happen, then their reward is purely based on company image alone, The products they gave out would barely dent the profits of Nintendo, but are nice reward to encourage people to return these kinda things instead of allowing them to be leaked, like the DS testing tools were. I bet everyone forgot about those tools that were leaked. You know what they did? Nothing special, all they did was test the features of the DS and make sure everything was working and this card would obviously do the same thing.
If anyone honestly believes testing cards could be used for hacking, they lack a clear understanding of what these cards actually can and can't do.
EDIT: Honestly I am skeptical of this story myself since I doubt Nintendo would even be that careless to begin with. Even though the cards aren't damaging, Nintendo really likes to keep them under lock and key, and always destroy them so they don't end up on the market. Making them highly sought after, and could be sold for a shit ton of money.
At best all they would be good for is collecting dust on someone's shelf, but that would be one really expensive dust trap.