This is not the version of the Plex server, but probably merely of the Plex Web app.
At which point of the guide are you struggeling? Which function are you missing?
Yes music library has witnessed changes in the past, so yes this looks different now. If this still works I don’t know, but you can try it out. The scaper should be also installable on unraid, as you have full access to the PMS config folder. I would start with this and see if it shows up in PMS. I am not sure if this still works as plugins (and scrapers) have also seen “changes” in the past. But it is worth a try.
I would work my way from there, maybe tagging only one single audio book just to try if things work.