Boeing decided to outsource to Spirit, Boeing is responsible to attest the quality of what Spirit is delivering for a Boeing product.
If Samsung/Apple would outsource their batteries to Megabattery Company LLC and those batteries started to randomly explode we would all be blaming Samsung/Apple for not doing proper QC. I hope we all hold Boeing to a much higher level of scrutiny than cellphone manufacturers.
It's even in the article: "Spirit AeroSystems, which makes the fuselages for the planes, referred CNBC to Boeing when asked about the incident"
[1] https://en.wikipedia.org/wiki/Spirit_AeroSystems
[2] https://www.spiritaero.com/