[dgb23]

I agree, it is talking about a specific type of grid, where each column gets to have a fixed fraction of the parent container:

1fr 1fr 1fr 1fr ... 12times or so

Many CSS frameworks will provide a default grid system like this, and many designers will think in terms of such a grid.

But the author suggests that this is too rigid. And in fact a frontend developer will then have to interpret the rigid grid to be more adaptable and have either multiple flex grids for different breakpoints or use CSS grid to express more flexible flows.