Think about it this way: lines are discrete, because they are separated by an HSYNC pulse that is used to fly back the electron beam from right to left. But the 'pixels' within a horizontal line are continuous: there is nothing in the video signal that delineates them.
To the number of horizontal pixels is essentially limitless (if you ignore the bandwidth limit and the CRT scan raster) while the number of vertical pixels/lines is always discrete.
To realign your decoder to interpret the earlier poster: their "horizontal resolution" is the frequency along the horizontal axis, i.e. the density of signal changes within one scan line.
To the number of horizontal pixels is essentially limitless (if you ignore the bandwidth limit and the CRT scan raster) while the number of vertical pixels/lines is always discrete.