I said nothing about the source of energy. I merely said how much energy it would need to be -- you need to find a source of energy that is essentially equivalent to a small star. Doesn't matter what it is, or whether or not we've found it yet -- this hitherto unknown source still will need to produce that amount of energy, or more, in order to make things feasible. The number I gave are entirely independent of the source of the energy -- it's a question of required output.
Given the comparison I made to stars, for that matter, one possible source of such energy already presents itself in the form of nuclear fusion. So in fact far from ruling anything out I've probably managed to hint at how it might theoretically be achieved!
Don't underestimate the scale of the challenge required to achieve interstellar flight. It's probably not impossible, but it's a huge challenge and you ought to be realistic about that. As I've just calculated, the energies involved are comparable to the energies of small stars. That's not a trivial problem and while we can hope that it will be overcome, there is good reason to suspect that it might not be. Current nuclear fusion experiments are producing energy outputs measured lower than kilojoules, so you would need something in the region of a hundred million million million times more powerful.