First, a look at man termcap:

       ch   Move cursor horizontally only to column %1
       ct   Clear tabs

(j::) actually joins with no seperator. The character following the j is used to capture the separator. Nearly any character or paired bracket can be used, but colons are common. Other ways to write this which might have been more obvious: (j''), (j[]), (j<>). To actually join with colons, something like (j.:.) would have been used.

Zsh agressively sets tabstops to every 8 characters. This is in a function which I trap to the WINCH signal, to set the termstops to 4 using the $termcap sequences rather than the external command tput:

.on_winch(){
	# tabs
	local -a s
	repeat COLUMNS/$1 s+=(${termcap[RI]/\%p1\%d/$1}$termcap[st])

	# final
	print -rn $termcap[sc]${termcap[ch]//(\%i|\%p1|\%d)}$termcap[ct]${(j::)s}$termcap[rc]
}
.on_winch 4
trap '.on_winch 4' WINCH