Its not so much the Empire, but isn't the gulf itself hard to traverse?
Either way, it is Kar Duniash' responsibility to figure it out, absolutely, either way. Or they need to redistribute the segmentums.
I just think its ludicrous, now that I think about it, and believe it should be retconned entirely as a big forgeworld-level fluff oversight.
Edit: Every map is a bit different, but it looks like Kar Duniash has a clear shot to Ultramar around the Gulf.
Warp routes being what they are though, I'm sure neither route is quite straight.