*sighs*
"Just add our key to your list of trusted keys, download this file and execute as root" is the default installation method.
Way down the page they mention you could also just download a zip and extract it to your home folder.
Nothing executes, nothing needs root, it's actually less steps.
But sure call it "manual install"!
(Tbf, I won't get automatic updates with that... but it's just eyecandy, I doubt the icon set will have a severe bug that requires an ergent update...)