Fairly certain that isn't true. On my wife's iPhone, she was never able to use voice to text while in airplane mode. Maybe that's changed with the latest 11 but it definitely didn't work on the X.
Go to Settings -> General -> Keyboards. If your language and phone support it, it should say "You can use Dictation for {language} when you are not connected to the internet." below Dictation Languages. It works since the 6s.