In English, I would never say completely new. I would 99.9% of the time say brand new. That sounds like they are saying it has all of its parts, but not necessarily put together yet. Does Duolingo accept brand new in place of completely new unless it still has to be put together that is. If you are driving a "completely new" car off the lot, I would expect it to be fully assembled and not in parts, so it would be brand new in my opinion.
You are assuming that parts are never reused in new things or are reused in new things so infrequently that you can assume that the new things you buy do not have any used parts. I doubt that this assumption holds up everywhere. Whenever and/or wherever this assumption does not hold, it can be a useful distinction (completely new vs. new, but having some used parts).