Maybe a small one, but controlling all the software for a device is definitely a type of monopoly - Nintendo has 100% control of what's allowed and permitted to run on the hardware. If I want to sell Switch users software, I can't without Nintendo's blessing because they have complete control of the market.

That’s not really what a monopoly is, at least in any useful sense.

Applebees has 100% control over the food that they put on plates inside Applebees, they can ban you from bringing in outside food and beverages. But we wouldn’t say that Applebees has a monopoly on anything.

Nintendo has total control over the software that runs on their hardware. Their hardware model is similar to the model that my microwave has. People are welcome to try to DIY mod their Nintendo device the same way I can try to DIY mod my microwave, but that doesn’t mean the government ought to be putting a stop to Nintendo’s “monopoly” on how the hardware they build is built and imaged.

They are being granted a monopoly on the bits representing the software they created. That is literally what copyright is.