This is hard to do since every CachyOS installer I’ve used since 2024-08 installs at least a bit of Gnome by default when you choose Plasma as your only DE. This is true through at least the 260426 ISO. I’ve not tried the 260628 one.
This seems to be an Arch thing because both of my Manjaro boxes and both of my Arch boxes do the same thing. I’m on one of my Arch boxes right now, and there’s nautilus, (which would be the offending 2nd file explorer the OP complains about) plain as day sitting in /usr/bin
Not only that, but plasma-login-manager allows me to start Gnome from the login screen. Try it yourself from the drop-down box in the lower-left of the plasma-login-manager login screen.
Perhaps (this is only a guess) a minimum number of Gnome components are included in a Plasma install in order to facilitate running GTK4 apps?