In his The New Testament: A Historical Introduction and The Historical Jesus, Dr. Ehrman suggests that Didymos Judas Thomas is presented as the author of the text of the Gospel of Thomas. He says in The Historical Jesus (pg. 47/41), “The author of the book explicitly claims to be Didymus Judas Thomas”. 

Here is what the author says according to Dr. Ehrman’s translation in The New Testament and Other Early Christian Writings: 

These are the secret sayings which the living Jesus spoke and which Didymos Judas Thomas wrote down.

This does not strike me as explicitly a claim by the author to be Didymos Judas Thomas. In fact, the author is referring to Didymos Judas Thomas in the third person. Furthermore, he continues to refer to Thomas in the third person throughout the text (v.13). 

Furthermore, in his work, Dr. Ehrman concludes from facts that (1) “Didymos” and “Thomas” both mean “twin” (Greek and Aramaic, respectively) and that (2) Syriac Christians considered Didymos Judas Thomas to be Jesus’ brother that Didymos Judas Thomas was considered to be Jesus’ twin brother. He then points out that this would raise something of a conundrum with other Christian beliefs, namely, that Jesus was the divine Son of God born of a virgin whereas Thomas was a non-divine son of a presumably non-virigin mother. He also points out that there is no extant explanation of this conundrum from the Syriac Christians. 

Here is the rub: Why think that these Christians thought Thomas was Jesus’ twin even if they thought that Thomas was a twin and that he was Jesus’ brother? He could have been the twin of another one of Jesus’ siblings. This would easily explain why the Syriac Christian apparently didn’t address the conundrum of the alternative.