I would argue that if the C program didn't "create" the sound that it was certainly the musician responsible.
Fundamentally though, you are of course correct. This was a mainframe in the 80s, so presumably there was an operating system and system libraries and they all played a part, so should we credit those developers as well? it's tortoises all the way down.
Composers have a 'musical imagination' and often write at the keyboard of (typically) a piano. Classical composers sometimes write music that is not 'idiomatic' for a particular instrument; adjustments follow. From the original article it seems as if the composer had a definite idea of what he was after and adjusted things until he got close.
What interests me about the original anecdote is the use of random numbers in the parameters, and the difficulty that the composer had in reproducing a particular 'state' of the sample.