I thought the point of the article was that if the person sending the text has iMessage enabled, there's no way for the author (the receiver) to prevent it going to the old device.
That was the author's assumption, but it's not correct. iMessage requires both the sender and the recipient to have compatible devices with iMessage enabled.
Even putting your iPhone in Airplane Mode will cause you to receive all your text messages via SMS rather than iMessage.
Even putting your iPhone in Airplane Mode will cause you to receive all your text messages via SMS rather than iMessage.