I really don't understand why some think it's a "rule" if it has so many exceptions.
Dune is far from the only scifi monobiome.
The planet Hoth, where the Rebels have their base, at the beginning of The Empire Strikes Back, is an ice planet. The planet Winter in Ursula Le Guin's The Left Hand of Darkness is an ice planet. There are dozens of others, many of which are documented here:
Think about why your examples work: We only see the base area and immediate surroundings on Hoth, and only for a short while. These planets work in Star Wars because they're each a single small part of a whole. The planets are monobiomes, but the story moves between enough of them that while it may look silly, the setting is still varied. It's very different to a story that takes place predominantly in one biome.
Note exceptions like Naboo: One of the few planets in the Star Wars movies where a story takes place across wider areas of a the planet. Suddenly there's a planet with varied biomes.
The monobiomes in Star Wars are monobiomes because there was no visual reason to expand, so they just made it the same in most cases.
In the case of Winter the planet is cold - in the middle of an ice age - but while the planet is all cold, it is not all identical. Ursula Le Guin explicitly drew maps [1], showing the glaciers, but also glacier free areas, mountain ranges, oceans, rivers. It's a cold world, but diverse within those limitations. It's not a single identical place. And the cold serves a purpose of setting the cities apart from the adversity of escaping over the ice shield.
I'd argue it's a great example of knowing to break the rules: Make it an interesting ice planet with stark differences between different areas. The problem is not specifically that something is an ice planet or a desert planet, but that something is uniform to the point that describing one place describes the entire planet. The easy way of avoiding that is to not make it an ice or desert or forest planet, and so if you need a tutorial on how to build your world, that's probably your best bet. The hard way is what Ursula Le Guin did with Winter of creating a diverse and interesting world within narrow constraints.
I'm still going through the link, but I think I can answer your question in the general sense. In creative writing classes you get a lot of rule of thumb advice similar to what's being talked about here. The reason it's a rule isn't because you're not supposed to break it. The reason it's considered a rule is to discourage students from using a particular theme, technique, or idea as their primary tool at the expense ignoring growth in their personal areas of weakness.
It's not assumed that the student will follow the rule once they've finished the course. But during the learning period they're discouraged from breaking the rule because the overarching goal is to have them develop a broad technical skill set that they can then refine back down into their own particular style.
Well, yeah, the fact that it's an overused cliche is a pretty solid argument against.
Dune is a carefully-thought-out world with a consistent ecology. Hoth isn't a "world" at all in the worldbuilding sense; it's a few miles of snow for a setpiece to happen in. And that's okay, if you only need it for half an hour and a setpiece. There's nothing wrong with choosing to write fantasy adventure with space opera trappings. But if you want to write science fiction set on a compelling world, you need to understand why Dune is interesting and Hoth is not.
