I'm guessing that at the time Dershowitz said so, the feds had not yet Mirandized him. Which makes sense since Bomber #2 was nearly incapacitated and couldn't talk. At this moment
it seems he can barely talk. The night of, he was in critical condition. He wasn't in any capacity to acknowledge his rights even if they had Mirandized him. (Waiving your Miranda rights has to be voluntary and knowing.)
Now, however, he has been Mirandized. Dzhokhar could certainly give enough evidence of an intention for terrorism (if that is a necessary requirement for death -- I'm not sure it is) in his answers to police. Or there could be other evidence. It certainly to me appears to speak for itself as terrorism.
I'm not a law professor, but I don't see how Dershowitz is right here knowing what we know now.