That’s not how resolution works. 720p is 1280x720 pixels, so 921,600 total. For a 5.7" screen, each pixel would be just under 0.0039" square. The angular resolution of a circular aperture (e.g. your iris) is given by the Raleigh criterion. The angular resolution required to resolve an individual pixel is 0.0039/d, where d is the distance from your eye to the phone.
The page I referenced gives the resolution limit for most people as 5e-4 rad and for completely optimal conditions as 2e-4 rad. So most people would have to hold the phone 7.8" in front of their face to see a pixel, and Carrots McGee, who has record-breaking vision and is sitting in a well-lit room with the phone brightness on maximum, would be able to see individual pixels at 19.5". These numbers are slightly smaller without rounding, but let’s be pessimistic and go with these.
Now, let’s see what Purism gave up by forgoing a 1080p screen. On a 5.7" 1080p screen, pixels are 0.0026" square. The minimum viewing distances for John Doe and Carrots McGee on the flashy battery-hungry 1080p screen are 5.2" and 12.9", respectively.
So, if you have world-class vision and really need that 13" - 19" viewing range, then yes, you’re missing out by having the lower resolution screen. For everyone else, there’s no practical difference.
Edit: as @kieran points out, the actual screen resolution is better than 720p. Mininmum viewing distances for 720x1440 resolution are 7.1" and 17.7" for normal and optimal conditions, respectively; the viewing distance window between “HD+” and 1080p resolution is less than 5" for even the most sharp-eyed people in the world in optimal conditions.