No defense attorney wants to allow a Defendant to testify when it will open the door to impeachment of their client. Moreover, an attorney is ethically bound to not allow his client to knowingly lie. Though I have never looked into Lawyer ethics of the 1880s, I imagine that similar rules applied then. That being said, it is not uncommon today or even in Wyatt's day, for criminal defendants to lie to the police when they are arrested, lie to their attorney concerning their case, or to even lie during their testimony.
I have no problem if people want to assume the the defense team would not allow Wyatt to lie about Tom having a gun. In fact, one would hope that the defense team would not knowingly allow Wyatt to testify falsely. Therefore, the issue is not whether the defense team would allow Wyatt to lie about Tom being armed and risk him being impeached, but, instead, whether Wyatt lied about Tom being armed in the first place. Which is the argument that I have asserted and maintained. You see, if Wyatt lied to his defense team they would not have known that his comments about Tom having a gun were false. Before an analysis of the defense teams actions can be done, we must first consider Wyatt's actions. About this issue, Casey has commented: "Recall that Earp was a law officer who had been around courts a great deal. He would be in a position to know that if his statement could be broken, it had the potential to send he and his brothers to the gallows. It defies sense that he would lie in a place where he stood the chance of being easily caught. . . ."