It means that the language as such - the codebases written in it - have a readability problem. If every project is written differently with opposing coding standards and practices, then it adds a significant switching cost when moving to a different project, debugging systems that includes libraries from various sources or reusing code.

For ecosystems where there is a "one way to do it", then picking up a random project or library from github and using or changing it is easy, as it is understandable in the same way as my own code; for C++ it is, well, different.

And this is directly caused by the language - the technical details of the language significantly influence the code ecosystem that grows around it. Lisps are another example of this - because it's simple to define commonly used stuff yourself, it results in every project doing the same things differently, adding a serious cost in readability and maintenance for anyone that's not the original author.