The point is that the fewer the number that can be shown to have died, down to “only two”, Peter and James brother of John, with “very high confidence”, the weaker the basis this gives to the claim that “they were all suddenly willing to die for what they saw.” And so the weaker the Martyrdom Defense of the Resurrection becomes.
This admission is well below the implication of “it is possible that not all the disciples (excluding John) were martyred” (which implies that most but maybe not all of them were martyred).
I would note that according to Tactitus the Christians (including Peter) died as a scapegoat for the Great Fire of Rome, not “for what [he] saw”.