His point is that there are monitors that are designed to work fine in portrait mode. Their primary intended mode is landscape, but it's not like portrait is completely ignored as a possible option by manufacturers. It's largely based on which type of panel the monitor uses, and since you didn't mention that, here's the differences:
TN Panels generally have horrible viewing angles and generally bad color representation (relative). IPS Panels generally have much better viewing angles and colors but cost more to produce.
Most high quality IPS based monitors have viewing angles in the ~170 degree range (landscape) or ~90-110 degree range (portrait).
TN Panels have other advantages like faster refresh rate (for the gamers) and generally being cheaper to produce, so most people have these just due to the cheap factor and they are good enough for a good portion of the population.
TL;DR: TN panels suck for portrait while IPS panels are much better for it, and since most people use monitors in landscape anyway, they see less need for the more expensive IPS panel based monitors.