We don't know who they people he brought over are. It's doubtful that they are people who've never done anything but write code for Tesla. Were I in Elon's shoes, I could easily imagine digging up some employees with prior relevant experience and asking them to just browse through the codebase and give a rough appraisal of how tight it is. You can tell a lot by looking at basic things like coding standards, state of source control, development environment, testing strategies, what kind of code reviews are happening, how modular the code base is, what the service architecture looks like, etc. I've worked in a number of industries and good code and tight systems look different from bad code and sloppy systems. He's probably not looking for anything specific, but rather to get some understanding of the state of engineering there.