Good question. I haven’t used custom hooks myself, but I believe you are correct. The alpm (Arch Linux Package Management) hooks manual states:

Hooks are read from files located in the system hook directory /usr/share/libalpm/hooks, and additional custom directories specified in pacman.conf(5) (the default is /etc/pacman.d/hooks).

So I guess the blog post means to say, that hooks are not supposed to be added automatically at installation of a package.