> fully customizable

There are all sorts of customizable. CachyOS' use of that word is rather inspecific. I guess it means the compilation flags are better customized for your CPU, plus it is easier to choose a kernel with a different scheduler enabled. So, "more customizable" in that sense.

Of course you could ask whether you could 'customize' your distribution not force you to use systemd. Most popular distributions fall flat on that one, I'm afraid...