You may be correct about the oath or lack thereof. I was unable to reead the entire Marvel run, so I may have missed it. I do know that in the movie, neither young nor old SE talked (other than the "Keis" in a fight scene). I think after Mr. Hama made the comment that he does not speak, the writers went with the spirit of the comics. It would be the same as the "Shadowed Snake Eyes" seen in the SE origin story. They wanted to keep the mystery of him alive.