Wouldn't that be explained by the next paragraph?
> The distinction between the two astronauts makes sense only when you reintroduce the rest of the universe. In the classic interpretation of general relativity, then, inertia exists only because you can measure it against the entire cosmic gravitational field.
This seems like a case of a spherical cow[1], the thought experiment is simplified down to only these two astronauts. No fabric of space around them, and so no inertia.
Perhaps it would have been less confusing if the thought experiment had used two normal people floating in nothingness? The presence of spacesuits makes it seem like they exist within some sort of reasonable environment.