You could add the commands described at Command completion to you .zshrc file, after the part which loads Zim:

# .zshrc

# ...

source ${ZIM_HOME}/init.zsh

autoload bashcompinit && bashcompinit
complete -C "$(which aws_completer)" aws

Note that I omitted loading compinit in the snippet above, because it is already loaded by Zim's completion module.