The principles of a grid-based design are still adhered to though. This isn't "gridless" in any real sense (other than maybe its handling in CSS?) from a design perspective. Maybe a better term is "adaptive grid" or something similar, which would make more sense in the design for something in a shifting and dynamic medium.
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.
