I don't think that's accurate. The defense has only identified one other person investigated by the police, which doesn't make for much of a "shotgun". I think the defense is probably doing one of or both of the following:
(1) The defense is trying to impeach the testimony of the aforementioned DHS agent. The unstated implication is that if the agent changes his opinion about the identity of DPR, then his opinions aren't very reliable and shouldn't be taken seriously by the jury.
(2) If the underlying reasons for this DHS agent's belief that Karpeles was DPR are still valid, then the defense could use those reasons to show reasonable doubt as to the true identity of DPR. Basically, "if the DHS agent doubted that DPR was Ulbricht, then it's reasonable you doubt that too!"
(1) The defense is trying to impeach the testimony of the aforementioned DHS agent. The unstated implication is that if the agent changes his opinion about the identity of DPR, then his opinions aren't very reliable and shouldn't be taken seriously by the jury.
(2) If the underlying reasons for this DHS agent's belief that Karpeles was DPR are still valid, then the defense could use those reasons to show reasonable doubt as to the true identity of DPR. Basically, "if the DHS agent doubted that DPR was Ulbricht, then it's reasonable you doubt that too!"