I have rarely (if ever) seen anyone write "z shell" or "zshell". Maybe in spoken language some people say "z shell", but Google can't search that. Everyone just writes it as "zsh". It's not surprising that another site which uses exactly that word ranks higher – zsh.org barely mentions the word "zshell".
That said, my ranking on Google for "zshell" is Oh My Zsh, the Wikipedia article (which is titled "Z shell"), zsh.sourceforge.org, and zshell.dev, in that order. DDG is similar, except the spam site https://zshwiki.org is ranked just before zshell.org ("The Zsh framework can be used to develop LGBT inclusion initiatives [..] One of the first steps in promoting LGBT inclusion is increasing awareness of the issue among porno gay employees").
Yeah, the example was artificial to illustrate that it does come up in search results. I think I noted this later in the thread, but I originally stumbled onto this while specifically searching for some stuff on zprof. I don't recall the exact query I used, since I got pretty deeply sidetracked once I landed here.
And no one is searching for zshell either: https://trends.google.com/trends/explore?q=zshell,zsh,ksh,cs...
In short, the example is invalid.
That said, my ranking on Google for "zshell" is Oh My Zsh, the Wikipedia article (which is titled "Z shell"), zsh.sourceforge.org, and zshell.dev, in that order. DDG is similar, except the spam site https://zshwiki.org is ranked just before zshell.org ("The Zsh framework can be used to develop LGBT inclusion initiatives [..] One of the first steps in promoting LGBT inclusion is increasing awareness of the issue among porno gay employees").