Let's make the hypothesis that Apple is going for a relatively mass market instead (affordable premium, a BMW series 3, but the reasoning holds also for cheap subcompact). Apple probably needs more than one production plant if it plans to launch internationally, but a single plant is enough for the US market and the reasoning before still holds. Actually, in this scenario Apple needs to work out faster the productions issue so being closer to home is beneficial.
This obviously is true if you think Apple will produce the car. But it doesn't make sense based on their production philosophy, and it will be a massive investment when there are "white label" car manufacture ready to be the Foxconn of Apple Car. The first name I could came up is Magna Steyr but if they go for low volume super luxury there are a bunch of small manufacturer in Italy with a good pedigree (Touring, Bertone) with the know how, the production capacity, and the brand that fits Apple needs.
I'm not saying you are wrong, even if my bet for the plant in Atlanta is for a European Automaker, I'm just saying that looking at the logistics, building a plant in Atlanta is not really effective for Apple. Probably tax breaks could change this balance, but I doubt you could have secrecy and tax breaks for a business of this size.
I haven't seen any news regarding this in any of the auto websites or related sub-reddits
The problem for Apple right now is that is so huge that any business idea is so small in scale that doesn't move the needle. I mean a 1 billion dollar idea is a great pitch, but for Apple is less the 0.5% in revenue, doesn't make the board level.
From this point of view there are very few industry where you could venture with such a huge scale. The automotive industry is one of them and appears to be ready for disruption. The problem as mentioned in the article is the incredibly low margin. I don't believe you could cut a lot on the cost of production, years of experience have optimized the process an the material that get into a car is that much, you could reduce them by a lot without impacting safety. Apple could try to put a premium on their car, and the margin on ultra-high premium car (Ferrari, Lamborghini...) are higher than Apple's margin. But the price is so high that the revenue is razor thin. The point is, a 30% premium on a device that costs on average 400$ is 120$, doesn't reduce the buyers pool by a lot. A 30% premium on a 20K car is 6K, it change a lot. And if you go in the premium segment is even more.
Final consideration, maybe the price Apple is aiming are not electric car but direct competition with Google's self-driving car. Or if you want car as-a-service. It is something not totally in Apple's DNA but it could provide hefty profit margins. Could be that the release date is 2029 and not 2019?