You use the term "Typically" which is typically true in the last 8 - 10 years. Before the first malware was placed in a rootkit it was developed by Unix for system Administrators. All a rootkit does is hide whatever is placed in it ie ... software, malware, underwear etc ... .
at Yen, I was thinking along the lines of the 2005 Sony *@#%-up where on their CD's they put Anti-Pirating Tools in Rootkits, which in turn let them be downloaded on consumers computers without being noticed by the user or any AV s/ware. I was wanting to get the device that allowed the Sony's tools to remain hidden. Still reading that link you provided.
I remember that fiasco. If you looked at the outer-most region of the CD (With a magnifying glass), you noticed an extra track. We would go in with a sharpie and "black out" that first track. Protection removed.
I think that was "Cactus Audio Shield"? By the way, CD, DVD and BluRay media is read from inner to outer, so the outer-most track is actually the last one. The idea of that crazy "copy protection" was that computers prefer to read the data track first if there is one. By blacking it out, the optical drive only sees the audio and works as expected.