That's completely irrelevant. Boeing can and does use many contractors, who have their own contractors and so on and so forth. Boeing is the company charged with ensuring these planes are safe, passing compliance, fixing any problems and so on. They are the company ultimately responsible for everything, and they must take steps to ensure that everything is safe.
If they choose to spin off parts of the company they don't like or use some contractors because they are cheaper - that cannot come at the expense of safety! It's their decision what contractors they use for what and how, and it's their duty to ensure ultimately safety. They must take every measure necessary to ensure this, and if they haven't - then they better begin to more closely inspect every component they get, or dump the idea of using a contractor for some components and bring it in-house, if there is no other way to ensure safety.
At no point can Boeing claim that it was't them - that's not an option.
Just look at shotty apartment construction as an apt analogy for why it must work this way.
