It's not just because demand might turn out to be soft. It appears that productivity has not been as high as anticipated, and as a result Tesla is taking a brute force approach to meeting their production targets, ie by throwing bodies at the problem.
However, it seems very likely that productivity will eventually improve, so Tesla wouldn't want to lock themselves into a workforce that is far more than necessary to meet their production goals. So I think it's more of an "all of the above" strategy, where Tesla is just trying to stay flexible so they can adapt as both the market and their production capabilities come into greater focus.
Yes, but if they end up with extra capacity wouldn't those people be able to shift into Model X production?