All they mean is that it won't output full HD - i.e. 1080p. A lot of people were - and still are - happy with a 720p image on a 42" screen. The majority of TV broadcasts are still at a resolution that's not full HD - and you don't hear too many complaints. Prior to the advent of HD, 720p is all there was. So long as you're not sitting unreasonably close to the screen, and you're not a picture perfectionist (I'm certainly not) I think you'll be very happy with it.
OnePlus 5 review