And saying something it 'unbelievable' doesn't actually make it unbelievable, there's this thing called hyperbole.
When characters literally cannot believe a thing, that is not hyperbole. That is purposefully depicting something as meant to be unbelievable.
I don't know why you keep bringing in real world analogues or elements not mentioned in the story, though. What Hotaru may or may not have done outside the story is irrelevant, because it
is not part of the story. And that's pretty much the entire point of the issue I take with it is. Bringing up possible external theories does not solve the basic problem of the story itself lacking. If the author meant for Hotaru to have practiced 3D a bunch before going to France, he's failed as a storyteller for giving absolutely no indication of it. If he didn't even consider it, the story itself is failing. Either way, the plot point is an issue. If the plot actually showed her hard work, and if it didn't constantly depict her abilities as miraculous in all other instances, I wouldn't have nearly as much of a problem with it.
And after rereading, Kou -still- calls Hotaru a genius (to her face I might add) not just a hard worker. So Kou is not saying "She's not a genius it's just hard work" she is saying she's "a genius that works hard".
Again, I don't know how well it came across in translation, but Kou explicitly says shes not the kind of genius that people think. Then goes on to describe how much work she puts in, and says her ability to keep up that work is really her true talent, and that all of her current skill is entirely just built up from her hard work. When she says she still thinks of Hotaru as a genius, she's saying that she used to think of her as a genius of raw natural talent, but now she thinks of her more as a highly-practiced master.
I'm estimating the 15 minutes based on the kind of practice they're doing; 15-20 minute quick sketches are the standard for Japanese drawing practice. Also the fact that they finish in the span of a single conversation. And end with breakfast being ready, so it's not like they could have been planning to go for that much longer.