There are plenty of people who can read code who don't work as devs. You could ask the same about testers, ops, sysadmins, technical support, some of the more technical product managers etc. These roles all have value, and there are people who enjoy them.
Worth noting that the blog post isn't just about documenting code. There's a LOT more to tech writing than just that niche. I still remember the guy whose job was writing user manuals for large ship controls, as a particularly interesting example of where the profession can take you.
Also, the primary task of a tech writer isn't to document code. They're supposed to write tutorials, user guides, how to guides, explanations, manuals, books, etc.
The other way around. For example the Python C documentation is full of errors and omissions where engineers described what they thought should happen. There is a documentation project that describes what actually happens (look in the index for "Documentation Lacunae"): https://pythonextensionpatterns.readthedocs.io/en/latest/ind...