That’s not really any different from how Airbus works (and explicitly so).
But I learned that Boeing literally outsourced the fuselage and that makes you wonder if they are really fit to build airplanes…
Boeing outsourced the fuselage to Boeing in Wichita, which had always built them. Boeing spun off that branch into a separate company now called Spirit, which is still basically Boeing except for its name.
Spirit is not just Boeing. Since the Spin-out Spirit has been buying many other companies and they are a supplier to both Boeing and Airbus. About 65% of their costumer base is Boeing. Spirit is a totally independent company.
If it's "still Boeing except for the name", that still smells funny.
They got something out of spawning a new corporate entity. Without a convincing TL;DR I think most of us will assume it's for reasons that sound good to Boeing business people, but not to outside people. (I. e. being a "new entity" to avoid existing labour contracts, or serving as a liability firewall in the event chunks of plane start flying off)