It's been almost 10 years, so I may be mistaken... but I think the director of Primer mentioned this in the DVD commentary.
EDIT:
Right, not really part of the plot. I just think I remember him (in the commentary) talking about some of the "trickier" aspects of writing a story about time travel, and I think he mentioned the fact that nobody ever really addresses the positional aspect.
I think the model of time and space in Primer is more consistent because it's a very specific kind of time machine.
If I recall correctly, you'd turn on the machine, wait N hours, go inside the machine, and in the next N hours you'd be going back in time and come out the other end at the moment the machine was turned on. So you would have spent 2*N extra hours, the machine would have always been in the same spot, so there'd be no problem with traveling through space.
There are still problems with the paradoxes, but that's part of the plot of the movie.
EDIT: Right, not really part of the plot. I just think I remember him (in the commentary) talking about some of the "trickier" aspects of writing a story about time travel, and I think he mentioned the fact that nobody ever really addresses the positional aspect.