Speaking as an aerospace Engineer, I don't buy it, or your argument either. The source article does not describe the motor you just described, anywhere, though I am curious, do you have any references where we might read more about it?

What the source article does say is "DARPA has confirmed that nuclear reactions will not be used to launch the craft from Earth and into space, but rather they will be used once in outer space to propel the craft forward."

Sounds eerily like Orion to me.

Again, the question has to be asked, how exactly are all of the materials of the craft to be transported into space?