What's more, a lot of the materials science and metalworking "lessons learned" (particularly when it comes to working with aluminum) at SpaceX are available to Tesla.
The key isn't being able to do it (as someone else said joining metals is a solved problem), it's being able to do it with high quality and in a reliable way. Do you think Space X joining techniques are optimised for making 100k joints in a factory environment?
https://electrek.co/2015/05/24/spacex-transferred-novel-weld...
SpaceX did not invent friction stir welding, but they did develop a mastery of the technique. Luckily, unlike you, they hire people who have experience in it. The strength of friction stir welds is one of the reasons they use it in the Model X and Model S vehicles, both which blew it out of the park regarding NTHSA safety standards.
Musk himself describes it here (for SpaceX):
https://www.wired.com/2012/10/ff-elon-musk-qa/
They even mention a few of the aerospace bits they have in the Model S:
https://www.tesla.com/blog/tesla-model-s-achieves-best-safet...