It wasn't really a plot hole: the Death Star was still under construction, and those were relatively clear paths for assembly ships to move in and out.
I have always supposed that the Emperor's plan involved drawing them to the Death Star's true location for two reasons: (1) because trying to trick the Rebellion into assaulting somewhere else ran the risk of the Rebellion detecting a ruse (many Bothans died getting them the information, as famously said); and (2) he intended to use the functioning main gun to catch the fleet in a crossfire (i.e. "it's a trap!") It seem to recall (though I might be misremembering) there was even a little dialogue questioning the validity of the information.
Of course, the Death Star wasn't really wide open at all: it had NO holes, due to the local shield generator on the surface, which was protected from a small sneak strike by a large ground force waiting for ambush.
What didn't make sense was that the Death Star would have no shielding of its own. Sure, it had to be made to look vulnerable, so having a shield generator on the surface as its (apparent) vulnerable point makes sense in that context. But the moment that thing came down anyway, the DS's own shielding system should have come up. But it didn't have any! -- when that should have been the first thing completed! (Unless those were destroyed by the crash of the Executor? I'm pretty sure that explanation isn't the film.)
I suppose the explanation could be stretched to include the concept that the Rebels might have found out there was a main shield, and then would have been suspicious of a orbital shield generator on the planet -- or at least would have figured (correctly) that destroying it would be pointless -- so the Emperor decided to make sure the DS looked vulnerable by leaking the information that it had no inherent shields.
I think it's important to note, in criticizing his plan, that by all appearances IT WOULD HAVE WORKED! He perfectly caught the whole rebel fleet in a crossfire. (I'm pretty sure in the novelization, there would have been mention of gravity generators to block hyperspace jumping, which was a thing introduced in Ep5, and heavily employed in the battles of the original true sequel trilogy story from Zahn about Thrawn, if not also in the X-Wing games which I also sort-of remember.) And he caught the sneak mission with a lot more ground power waiting in ambush.
His whole plan would have worked, aside from possibly still losing to the Skywalkers, if Luke hadn't 'forced' C-3P0 to be a god to the local carnivorous teddy-bears, earning their alliance!
To this evaluation, may I add the implicit agreement of Zahn himself, who had his strategic and tactical mastermind Thrawn complaining, not about the Emperor's plan per se, but about building another Death Star at all instead of several more fleets based around Super Star Destroyers. (He mourned the loss of Vader's flagship, the Executor, more than the loss of either DS.)
Anyway, nothing matters anymore, because we now know that anyone could have programmed a hyperspace missile at any time to destroy either of the Death Stars, or practically anything else, whenever they wanted to.
