Ok, I found this link which is interesting and agrees with the "Batteries bottleneck" version

https://insideevs.com/musk-says-biggest-model-3-bottleneck/

> “To date, our primary production constraint has been in the battery module assembly line at Gigafactory 1, where cells are packaged into modules.” “We had to rewrite all of the software, from scratch. We managed to write 20 to 30 man-years of software in 4 weeks.”