Dr Nic,
For me, it was gcp sdk's auto completion that breaks the above stack
overflow post instructions. I added the following to my .zshrc *before*
gcp's auto completion loads:

_cf-cli() {
# All arguments except the first one
args=("${COMP_WORDS[@]:1:$COMP_CWORD}")
# Only split on newlines
local IFS=$'\n'
# Call completion (note that the first element of COMP_WORDS is
# the executable itself)
COMPREPLY=($(GO_FLAGS_COMPLETION=1 ${COMP_WORDS[0]} "${args[@]}"))
return 0
}
autoload -U +X compinit && compinit
autoload -U +X bashcompinit && bashcompinit
function compinit() {}
function bashcompinit() {}
complete -F _cf-cli cf

Which let both the gcloud and the cf work. The lines in gcp's sdk file
that break things are:
autoload -Uz compinit bashcompinit
compinit
bashcompinit

I did my cf code first and then redefine compinit / bashcomptinit to
no-ops since I already loaded them. I'm sure there are more elegant
solutions, but I don't want to fiddle with the gcloud distributed file
google-cloud-sdk/completion.zsh.inc.

-Matt