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.
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.