Imagine assembling a car engine without any QC process, perhaps just visual inspection of the parts. The end users (whoever will drive the car) will be the actual testers.
Because users always report bugs...
If you've ever been frustrated by the package manager Conda and the UX of its command-line interface ...I'm sorry. I tried.
Well, if you refuse to write tests and you are scared to make new changes without them... then you can't push new fixes at all. Which is apparently fine for some organizations.
The problem is, that there's no way to generally quantify "too much" or "not enough" when it comes to tests and documentation.
Even "harmful" isn't a well defined term on its own and needs to be properly defined on a case-by-case basis.
The statement is a platitude without any substance behind it and you can replace "testing and documentation" with just about anything and come to the same result:
Vitamines and calorie intake are both essential but too much of either becomes harmful.
> Individuals and interactions over processes and tools
> Working software over comprehensive documentation
> Customer collaboration over contract negotiation
> Responding to change over following a plan
The things on the right are only useful in service of the things on the left.