Well, he might have been on the other side of the moon for all we know.
Maybe? But it doesn’t make visual sense. You shouldn’t have to rationalize it.
You also shouldn’t think about it at all.
There must be more Imperial garrisons than that one platform.
Why? They’re just protecting the shield generator.
Why would they need platforms on the other side of the planet? Why would Vader arrive there & then have to travel all the way around the entire planet to get where he was trying to go? He knew what the Rebels were up to & about where they’d be.
Because the Emperor said there was an entire legion waiting for the rebels and that was the entire plan. There were walkers, speeder bikes and even the scout trooper said there was a base. It wouldn’t be that far fetched that Vader was giving latest info on the rebels or something before meeting Luke if you have to wonder where he came from in the first place.