While they’re pretty certain that the speed of light is the practical speed limit, it assumes that the fastest way between two points is a straight line, which may not be the case.
The way around the problem begins with the assumption that instead of two different things, time and space are two dimensions of the same thing. Manipulate one and the other changes as well. And such manipulation is common, caused by mass-gravity.
That is, think of the Einsteinian, “grid” model of space, with massive objects causing gravitational distortions to space, the ultimate massive objects, black holes, causing such a great distortion that not even light can escape from their gravity well.
And the model gets much more interesting when you consider that the reality is 3 (or more) dimensional, not 2 dimensional.
In any event, the gravitational distortions are of both space *and* time.
Now extrapolate that idea to the universe as a whole. From the point of view of a traveler between two points, if you are traveling at 10 mph, it will take 1 hour to reach your destination. If you travel at 20mph, it will take a half-hour. This is the typical view. But look at it from a different perspective:
If, while going at 10mph, you alter time so that you arrive at your destination just a half-hour later, relative to you, since your speed is the same, you must have reduced the spatial distance traveled, *whether it appeared to have been reduced, or not*.
Yes, that is simple length contraction, but I don’t see how it helps us get practical interstellar travel without getting near the speed of light. After all, the contraction effects are pretty negligible until you start getting to at least significant fractions of c.
We tend to constrain our thinking about these things in specific ways. Where we think of ourselves advanced, another civilization may see us backward at they same time we’re far advanced of them in other ways.
(Thank you Harry Turtledove and your short story “The road not taken” for that line of thinking.)