It is very unfortunate that safety critical software is not developed inhouse. The direct communication between engineering, development, testing and management

It is very unfortunate that safety critical software is not developed inhouse. The direct communication between engineering, development, testing and management is usually quite important. But this is never an excuse even if this software had been delivered from a contractor it is the duty of Boeing to write specific requirements and to test them, while making sure that the requirements cover all forseeable eventualities. This isn't frontend development. You do not just ship software some third party wrote for you.