I don't think that those additions make anything clearer, I'm afraid.

"child python process" would seem to imply raised in the new python running in the child process, which is clearly not true.

Hmm.  I seem to remember having this discussion before.  The current statement is in fact exactly correct.  When shell=True the new program being run is 'sh' (or similar).  So the error message is coming from the program being run successfully by subprocess.  End of story.

So, we could add a clarfication: "If shell=True, the 'new program' being run is the shell, which may generate its own errors based on the shell command passed to it (such as command not found).  These will not result in exceptions."