Do Twins have a "Secret" Language?

Chapter eight is about language, thinking, and reasoning. This chapter of the book includes how language works, the relationship between language and thought, and the relationship between thought and reasoning.
A portion of the chapter explains how children learn a language. The book suggests that the language-learning process begins while the infant is still in the womb. The chapter highlights a study that has shown fetuses are able to recognize characteristics of their mother's voice and her native language once they are born. However, new research has found that babies learn to talk by reading lips. Infants may recognize characteristics of a language, but in order to learn how to speak it, they have to figure out how to shape their lips to make the particular sounds they hear. Read more here.
A portion of the chapter that I found interesting was the part about special languages between twins. It reminded me of a viral YouTube video I had seen of twin boys that seemed to be speaking to and understanding each other. However, the book went on to explain that twins do not have a "secret" language; they are only attempting to use their native language, but with poor articulation and pronunciation errors. What do you think? Is the information in the book correct? Or do infant twins have their own language?