[V_Terranova_Jr]

This is likely an oversimplification. There are multiple organizations, even within Boeing Defense & Space, that write their own flight software. All stovepiped and largely working in parallel. This doesn't even include the commercial folks, infamous for the 737 Max. My understanding is that the St. Louis teams are better regarded, and the folks that worked on DARPA HACMS deserve some credit, but they seem to be outliers. Boeing's culture doesn't seem to prioritize modern software development methods or software rigor on the whole. Functional testing should be the last layer of bug-hunting techniques, not the first or primary. The issue seen on their capsule didn't surprise me at all. Other BDS software groups use utterly outdated software development methods and we should all be a little bit worried.

[JshWright]

Obviously it's a simplification; comparing and contrasting the approaches of SpaceX and Boeing would require several walls of text...

At the core of the issue though, the process that SpaceX pitched to NASA (and NASA approved) involved quite a bit of actual hardware testing. Boeing's plan (also approved by NASA) relied much more heavily on simulation, modelling, and other sorts of process validation. For instance, Boeing did not perform an in-flight abort.