TK is a journalism joke. In that field, placeholder headlines used to commonly be written in as “Headline TK” or “TKTKTK” etc. It comes from back when newspapers were typeset physically before set in lead to be sent to the printer. It is short for To Come. Many editing marks are misspelled on purpose to make them stand out from the final text. “TK” is an unusual pairing of letters, so it stands out.