Good question, thanks!
When you're missing skill to complete the assignment and you made it clear for the company then I think what they are looking for is for you to demonstrate that you're capable of learning that skill on your own in a short time.
6-8 hours deadline is very tough in my opinion, but what can you do about it, eh? Those are the rules of the game, you have to follow them I'm afraid.
Would I do that? If I wanted the job I would do my best to complete it. Even if I missed a deadline I'll send it back anyway. I would tell them, that I had to learn a few things first of course. If I were an interviewer I'd respect that attitude.
Btw, if you learned something new while working on the assignment, it's not a bad assignment after all. It's better than simply wasting your time on making yet another standard task, right?
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