My dad works in aerospace. In no field or world does anyone actually check every little part.
You create a process, you test that process so that you understand its limits, and then you make sure to follow that process.
Now I don't know if 3D printing rocket parts actually works and I have my doubts but this startup is currently testing the process and they will figure out its limits. That's the whole point of R&D.
For context, 3D printing rocket parts is incredibly common, even student teams often use printed nozzles and such. The linked article is about printing a whole engine as a single piece, which is a different beast.
You create a process, you test that process so that you understand its limits, and then you make sure to follow that process.
Now I don't know if 3D printing rocket parts actually works and I have my doubts but this startup is currently testing the process and they will figure out its limits. That's the whole point of R&D.