I am sure someone with a better memory than me can explain how there never has been a consistent time frame for the Robin Hood legend. The ballads were written at different times and the characters were added from different sources in later telling. The historical Richard the Lionhearted rarely has any influence on the time frame of these stories.
The most interesting thing about this announcement is how well CW sees Arrow doing. Obviously this would be a very different show. But clearly the timing of this is based on Arrow's success. In these days of fragmented audiences it hard to tell what is considered a true success anymore.