I'm not saying that borders are always wrong, but you are using a horrible example as an argument in favor of borders.
There are no borders in any of the examples in Josef Müller-Brockmann's Grid Systems except for in the pictographic signs used by the the Swiss Federal Railwaws, but they are not part of a grid.
Well, there are also borders, or rather literal guidelines, in all the sketches visualizing the grid, but they are of course only there as a tool. In all the grid layout examples the grid is implicit. All of them.
> Unnecessary borders create visual complexity that increases the load on perception, which reduces cognition
That's difficult to believe in if you remove the word "unnecessary". Logical grouping of similar components in a UI using a border can only help group them. The framework for grid-based design was laid out by Josef Muller-Brockmann and heavily influenced the swiss/intl design language.
You now need to define what constitutes "unnecessary" :) otherwise its talking past each other.