Isn't that an argument that everything should be written in shell script?
It's an argument that everything which reasonably can and should be written in a shell script (that is, without compromising security or performance) should be. A shitload of what makes a modern Linux go is just scripting. Sadly, many of them are python scripts; shell scripting will do the jobs they do without exception, but people jumped on the new shiny (like they did with perl, as well) and that results in a system where you have to understand three scripting languages to maintain it, not just one. Clearly scripting is not a panacea. You have to understand what you are doing.
People often argue that shell scripts are slow, but they aren't if you have free memory, because process creation is cheap on Unix. Creating new shells, firing off all those inlines, is as cheap as thread creation on Windows. It's not a problem these days, when RAM is basically free.
Everything which can reasonably be implemented as a very small shell script absolutely should be. Anything which legitimately needs to be written in C absolutely should be. The right tool, not just a random tool.