That's not how writing works. An author -- even a good one -- puts stuff into the story to make things make sense. If the writer really wants the villain to show up and be threatening, then disappear for a period of time to give the heroes time to respond and plan and do whatever they need to do, then the writer needs to insert justification. Having the Admiral arbitrarily show up 9 weeks before harvest just so there's that time buffer is no worse or better than any other justification, as long as it makes sense in-story. The buffer in Rebel Moon makes sense as far as it goes, but it's not like it's some perfect manifestation of narrative. It's just a gimmick to give the heroes time to act.
Yes, it would be nice if the story provided some reason the Admiral wanted to starve this particular village aside from "my troops need food." And that leads into the next...
Even though he really knows he doesn't need it. Or should know. Or if he does need it, the story hasn't explain how or provided any justification for how that could be credible. His troops don't look emaciated. The story shows them trying to violate one of the village girls, not one of the granary stores. There's no mention of a food crisis back home.
I mean, at the very least, the Admiral should have insisted the villages use modern, high-tech methods, conflicting with their ideology. He doesn't even do that. It's almost like he doesn't give a crap about food, and was just using that as an excuse to exert control over a remote commune of rando farmers. Which would be fine, but jeez, did we need to spend 15 minutes debating wheat bushels then? It creates the impression that the writers don't even know what their own story is about.