I’be been a software engineer for nearly 2 decades and have been involved in multiple technical due-diligence endeavors. At best, you’re just grokking the big-picture and looking for any major red flags. Getting involved in individual code reviews is not a useful exercise outside of understanding a team’s SDLC and various coding practices - for an org the size of Twitter all of that stuff should have been documented and shared during due diligence (which Musk waved) - injecting outside engineers into the code review process is just an expensive and sloppy way to uncover what could otherwise be gleaned in an easier fashion.
I do see one big red flag for Twitter: it now has leadership that doesn’t trust the engineering organization. For a technology company that can be fatal if not resolved quickly.