Bowser: “We still believe that what makes our games special is our developers, their artistic capabilities, their insight into how people play. So, there’s always, always going to be a human touch, and a human engagement in how we develop and build our games.”
What you’re describing is fine tuning, not model creation.
You can train diffusion models from scratch, even on home hardware, using open source software. It is well with the capability of Nintendo to do this with their own artwork.
Adobe did, they created their models from artwork licensed from artists specifically for training their models.
There’s no reason to think that Nintendo would use public diffusion models when they can train their own and have a model that more accurately reflects their style.