What that means is that alliances would be able to effectively lock the jump gates leading into (not out of) a system.
There's your problem. To stop someone getting into
your system, you have to stop them getting out of
the neigbouring system, at least on that gate. What if I'm surrounded by hostile alliance sectors? I'm trapped. Weee, fun game.
In any case, with player-owned structures coming, a warp disruptor with minefields, turrets, etc. will achieve a similar thing - that is, a blockade that takes a lot of effort to put in place and to get through, but that doesn't require people sitting on their bums for hours on end.