They wrote their own language (C) too. They invented a new language because the current crop of languages didn't suit their needs. Your argument ignores the parts of history that are inconvenient and highlights the ones that you think support it.
"Although we entertained occasional thoughts about implementing one of the major languages of the time like Fortran, PL/I, or Algol 68, such a project seemed hopelessly large for our resources: much simpler and smaller tools were called for. All these languages influenced our work, but it was more fun to do things on our own."
People arguing for UNIX/C, also tend to forget the decade of systems languages that predates them, starting with JOVIAL in 1958.
They also forget that after coming up with UNIX/C, they went on to create Plan 9, which was supposed to use Alef, abandoned its designed, later acknowledege lack of GC as an key issue, created Inferno and Limbo, finalizing with contributions to Go's original design.
"Alef appeared in the first and second editions of Plan 9, but was abandoned during development of the third edition.[1][2] Rob Pike later explained Alef's demise by pointing to its lack of automatic memory management, despite Pike's and other people's urging Winterbottom to add garbage collection to the language;[3] also, in a February 2000 slideshow, Pike noted: "…although Alef was a fruitful language, it proved too difficult to maintain a variant language across multiple architectures, so we took what we learned from it and built the thread library for C."[4]"
I think when Fish shell announced the Rust rewrite, they especially highlit that, in the form of "being more attractive to contributors" as one of the reasons.
