Tbf we had nuclear [fission] pulsed propulsion technology in the 1960s in roughly the same way we had nuclear fusion in the 1960s: we had an understanding of some basic principles of physics and some calculations that looked broadly plausible

The problem with using thermonuclear explosions to produce that level of thrust are more "but can we actually do it safely" than "but can we do it in a way that generates more that it takes to produce", but that's a not inconsiderable problem even without all the international agreements about where we shouldn't be exploding nuclear bombs...