Well the Dan Plan is looking to show that it is about training. But I should also note that the theory isn't that you stop improving at 10,000 hours, but that's how many hours it takes to be an expert.
Maybe Go is an outlier. I don't know enough about the game. But I'd be surprised that someone who did 10k hours of deliberate practice wouldn't be pretty good by most metrics of the Go community.
Go is likely similar to certain parts of language acquisition and musical talent in that there's a critical period involved. If you wait until you're an adult to start, not even 20k hours under a great teacher will give you real mastery. AFIK, every top player started training as a child.
Maybe Go is an outlier. I don't know enough about the game. But I'd be surprised that someone who did 10k hours of deliberate practice wouldn't be pretty good by most metrics of the Go community.