I don't get this part. The language you write something in has little to do with how useful the software is. I've had years writing fulfilling software, and years where I was just spinning my wheels.
The author was mostly referring to his efforts to improve the language, not to write some app or library in C++.
However, it's not necessarily true. Working with large codebases, working on language design questions, committees, everything grows experience that can be used later. It's just not clear now how, as requirements and work in other companies or in FOSS may be very different than that C++ development.
The author mentions Go as a positive experience. If the ego could be left at the door, Go could be a good language to make a new home in. Either that or Haskell, though with much more investment in all layers.
The language and tooling can have very much to say for overall development and maintenance. Also architecture and design matters, as well as changing requirements. It's all very simple or complicated, depending on unknowable X-factors that often need to be decided up-front.
That's not at all the point being made...