This made me laugh

the commenter misquoted, the rest of the sentence is "... iDevice as an extra display for your Mac using the Lightning or 30-pin cable."

Actually a bit simpler, Business Insider released the article to fill their timeline not Duet's launch date. The app is still awaiting review and with the app store being so busy this season it's hard to give a hard date. Expected to be released later this week.

Duet is still waiting in line for approval in the app store. Launch is later this week.

you chopped off this part "...iDevice as an extra display for your Mac using the Lightning or 30-pin cable"