Yeah, I remember this myself, when they were producing just the Model S. They made quite a big deal about 1. they were amazing stamping machines that they got cheap from a shuttered car mfg (can't remember who) and that they were designed to stamp steel, so stamping aluminum was a breeze. We actually got to see some of the stamping.. very cool.Currently on the Fremont factory tour you are shown stamping presses (in the same area as the enormous press that has been used for S panels since 2012) that the tour guide says are used for stamping Model 3 panels.
The story of Tesla operating a Model 3 body line in Detroit is suspect.
Much of the early production 'tuning' was spent getting the stamps just right. Somehow I doubt they would do that in another state, then get it figured out (without even local car assembly) then ship all the machinery to Fremont.
Separate question, are the body panels of the Model 3 aluminum?