I got it solved in the meantime. It was my misunderstanding. I thougt that the Neat grid framework would automatically stack defined elements when reaching breakpoints for smartphones and tablets.
But then I saw in the example files from the Best Practices for Front End Development workshop, that one needs to define breakpoints in a _grid-settings.scss file, and change the behaviour of elements like article when reaching the defined breakpoints.
In the meantime, I’m already designing my second website with Neat and Bourbon, and I gotta say they’re awesome tools that I wouldn’t want to trade anymore.
I’m really looking forward to those videos of the Best Practices for Front End Development workshop.