Some Tesla engineers will be better than Twitter engineers and vice versa, but the Tesla engineers aren't there as a bastion of better software engineering.

They're a "trusted" external party that Elon can use to get an unbiased assessment of what's going on internally at twitter and get a better picture of what's valuable and what's not. If you ask a senior engineer at Twitter the same questions you'll get a very different set of answers, not necessarily wrong answers but a different perspective. Unless Elon's an idiot he'll be doing both.