Where is the link to Tesla saying "customer production won't start until January" ?
Here is the link, it is just going to happen, stuff like this and especially of the magnitude that Tesla wants to ramp production in a plant that is still under construction on schedule is very very very hard to do, so a one-quarter slip is perfectly understandable.
However, I believe that a slip of two-quarters or more is not out of the question. I find it interesting today with everyone reporting their August sales/deliveries that the silence from Tesla is almost deafening.
I went flying a few weeks ago around the valley and flew over the Tesla plant in Fremont. If you look closely, you see that stuff is happening everywhere outside the plant and there is all sorts of construction going on. Check out in the lower left of the photo there is a 10-20 story building going up there that is going to stage all the parts for the cars being built all by robots. It isn't even built yet. Not shown, the massive warehouse facility in Pleasington/Livermore, and of course the Gigafactory. Don't forget staffing, training and all that HR stuff.
This is why I believe that Tesla is rolling the dice that all of these projects going on are going to mesh well enough to produce 100K Model 3s next year. This I gotta see.