I’ve never looked at that grid before (and I hate grids - although I have used bootstrap a number of times) but it looks to me as though what they meant to say that was a row in the phone layout will consist of 4 columns so in the example the first two divs (col-2-phone) will fit in the phone width and the third div col-4-phone will sit underneath and full width.
If you wanted all three divs in one row for mobile then you would need to set two of col-1 and one of col-2 but that would leave you with very narrow columns across the screen. A better approach would have been to make each col-4 for mobile so they stack vertically as two columns are two small. Indeed in tablet you have words sticking out of the side columns so a bad example all around I think.