Here's a short 2014 paper from McIlroy himself discussing his early thinking back then: https://www.cs.dartmouth.edu/~doug/sieve/sieve.pdf

TL;DR: McIlroy was applying the concept of coroutines, which was described by Melvin Conway in 1963. Two processes communicating over a pipe are basically coroutines, except instead of passing structured data they're just passing bytes.

A Unix pipeline is a set of multiple coroutines. In fact, Tony Hoare's 1978 Communicating Sequential Processes paper cites the UNIX shell[1] for the concept of coroutines, and discussion of coroutines figures prominently in that paper. See https://www.cs.cmu.edu/~crary/819-f09/Hoare78.pdf CSP basically models the behavior of a large set of coroutines.

AFAIU, Erlang was partly inspired by CSP. You can draw a straight line from coroutines, through Unix pipelines and CSP, to Erlang's processes.

[1] Specifically, Ken Thompson's 1976 paper, "The UNIX command language." See https://archive.org/details/theunixcommandlanguage