Hi - author here. Completely agree - if you want to make much more complex data graphics, learning the nuts and bolts of d3 is vital. You certainly can't escape that.
But I would add that not everyone wants to make more complex ones. For a lot of needs, 'using a library like this will only get you so far' is often exactly what folks need. Many of us just want to make charts that work really well, and are really simple to use. We grew tired of having to re-implement the same ideas over and over again for new dashboards and reports. Hence this library - a way of simplifying what we've already done, and making it easier to add new features in a principled way.
But I would add that not everyone wants to make more complex ones. For a lot of needs, 'using a library like this will only get you so far' is often exactly what folks need. Many of us just want to make charts that work really well, and are really simple to use. We grew tired of having to re-implement the same ideas over and over again for new dashboards and reports. Hence this library - a way of simplifying what we've already done, and making it easier to add new features in a principled way.