I think his point was simply that writing such CSS is hardly complicated. I like this abstraction and think it will be pretty handy for Compass users (of which I plan to become one), but this mixin basically reduces ~8 lines of CSS to two.
1. Button reset - to get browsers to the same starting point
2. Rounded corners, Text shadows, and Gradients + vendoring
3. Background image fallback
4. Padding, Text size, line-height, etc
5. Default, Hover, and Active states for all styles
That's a lot of css to write, effectively reduced to a single mixin. Of course, it's also great that you can use the bare minimum html.