That's the point of this piece. For any non-trivial edit on a real project with real deadlines the source code is effectively useless, because no one has the time, the resources, or possibly even the inclination to fix bugs, do full-coverage testing, or make custom modifications.
So you have to take the internals on trust. Which is a ridiculous situation when so many packages are created as hobby projects with - literally - no guarantee of performance or reliability.
I realise it's hard for FOSS advocates to understand this, because it's a fundamental flaw with the FOSS philosophy. The benefits are "obvious" to crusaders, but the objective reality is that large swathes of FOSS are full of casual or hobby code that barely works, has gaping security vulns, and/or is nowhere close to being robust enough for production.
"Make software simpler" is a good goal, but hard to do. Other solutions are also possible. They're hard too. So it goes.
But there will be no solutions at all until the FOSS community starts dealing with professional reality instead of relying on free-to-tinker-without-consequences rhetoric - and understands that there are real problems that need real answers, and not just more "Clap Louder" and "At least we're not Microsoft".