So the thing is all of the screens will take an HD signal and convert them to native resolution which happens to not be HD in this case. Things like this are incredibly useful when you need to monitor alot of feeds through some form of switch. Say you have a 24 video feed switch and you get 24 of these little monitors and one big 1920x1080 monitor in the middle which can select the feed from any of the 24 little monitors. You can now monitor all at once, but zoom in on what is important at a given time (Think security monitors, TV station control rooms etc.)
HD input ability on an SD monitor is perfectly valid when using a monitor like these. Where they are referring to "monitor" as something that is "monitoring a feed", instead of referring to a ""monitor" like a "computer monitor".
