Well not quite everything. I see a new post editor but what about the underlying WordPress? I know Magento 2 isn't the bastion of modern web development but that was a definite "changes everything" moment.
It breaks most custom UI functionality that people used to implement features with the old editor -- like custom meta boxes. It's a pretty rough for people with sites already in place for a while.
If you’re talking about the core of the project of course they’re not massively changing that, but in the article it mentions that Gutenberg is implemented with React and other modern web technologies.