Some Emerging Possibilities For Space Propulsion

Pretty much.

Bob's argument is (more or less) that it's easy to set up a thought experiment in which you imagine setting up closed causal curves, but that the devil is in the details.

Earlier in the paper, he argued that the new physics will still have its own causal structure, in a well-defined sense involving initial conditions and differential equations. If you start with complete and well-behaved initial data, his argument is essentially that the state of the system will always evolve forward in time in a consistent way (and in particular, there's no way for closed causal curves to arise out of nowhere).

So those last paragraphs that you're asking about attempt to interpret that conclusion in the context of a thought experiment aimed at creating a grandfather paradox. Geroch's answer is to say "the details will depend on your precise model, but the math makes it unavoidable that something about the model will make your plan impossible". Maybe the experimenter's normal-matter hands just can't ever move fast enough to push the special fluid past light speed. Maybe FTL-fluid is explosive when it comes in contact with normal matter. But the ultimate mathematical conclusion must hold.

(I won't swear to have worked through every step of these arguments in detail myself, but Bob Geroch is a world-class expert on this sort of thing. The one caveat that I'd add is that I think his arguments here are classical, and that bringing quantum mechanics into the mix might change things. It's possible that it would only change things in topologically disjoint sectors within a general-relativistic spacetime, but something like that might conceivably escape his premises.)

Why not a fission fragment rocket? We could send a probe to the centauri system with current technology.

https://en.wikipedia.org/wiki/Fission-fragment_rocket

I have a hypothesis, unsupported of course, that everything has a fixed spacetime velocity. As beings with mass, we perceive the universe as having three spatial dimensions and one temporal dimension. We move relatively slowly in the spatial dimensions, and we find it easy to change direction. As a result, we are hurtling incredibly fast in the temporal dimension.

A photon, on the other hand, moves incredibly fast in the spatial dimensions, and finds it difficult to change directions in them. From its perspective, the spatial dimensions might as well be one dimension, represented by its vector of movement in space. It moves relatively slowly in our temporal dimension, however. And that leads to the question.

What if we only perceive time as a single temporal dimension because we are moving so fast through time that we have so far been unable to perceive any other temporal dimensions or change temporal direction?

It may be that if a human were to accelerate close to the speed of light in the known spatial dimensions (thus decelerating in the temporal dimension) it would become possible to perceive additional temporal dimensions, and maybe alter one's course through time. Our current understanding of causality assumes that one's movement through time does not change direction.

If time does have additional dimensions, a massless particle could execute a u-turn in the temporal dimensions, and it would appear to us as though it turned itself into its antiparticle. If the u-turn was in any way imperfect, the particle probably could never intersect any point in spacetime that it has ever previously passed through and interact with itself in a way that could create a paradox. Or maybe all electrons and positrons really are the same particle, endlessly interacting with itself as it bounces around in spacetime, and all paradoxes become self correcting, since causality flows in multiple directions.

TL;DR: Our assumptions about causality may be wrong.