This is all probably because the version of the script from mid-March 1982, before the final round of rewrites on the Endor sequence, reads a bit different here.
Note that, in addition to the Imperial facility being much bigger and more active than it ends up in the final film, there’s no description of Vader exiting his shuttle. Instead, the viewer is supposed to discern that Vader obediently arrived on the moon immediately after his conversation with the Emperor earlier, and has been waiting around for Luke to turn himself in.
Frankly, I think it makes more sense that way, and avoids the same “We must always see Vader travel between locations” thinking that lead to the addition of some things to the end of ESB in the SE. And the musical cue could work with just a better establishing shot of the Imperial facility.