The voyagers are nuclear powered. They have RTGs powering their electronics and heat from those devices has changed their trajectories. The concept of a nuclear reactor powering an electric thruster isn't very far fetched. If we were going to go anywhere outside the solar system, that is the sort of nuclear power we would use.
Sein is probably referring to something running quite a bit hotter and likely exciting hydrogen atoms or other expendable fuel sources. I think these might have been looked at for purely interplanetary/solar travel in the past, but written off for the dangers of lifting cargo to orbit (think about the failure cases!) and also the general cost compared to alternatives.
If we had mining in lower gravity well areas (such as the asteroid belt) yielding that kind of fuel it might make a lot more sense.
Specifically, projects Orion and Daedalus are based on nuclear pulse propulsion[0], which is essentially detonating thermodynamic bombs behind the spacecraft. It's one of the only realistic options we currently have to do interstellar exploration. There was a discussion on HN not too long ago[1].
The voyagers are nuclear powered. They have RTGs powering their electronics and heat from those devices has changed their trajectories. The concept of a nuclear reactor powering an electric thruster isn't very far fetched. If we were going to go anywhere outside the solar system, that is the sort of nuclear power we would use.