Hi,
i also remember to habe reed the hd things some were. But i‘m pretty sure they meent 720p and said it might will be full hd. I quite sure of this because it seams
pretty common to sell 720p as hd, but my personal undersanding is hd = 1080p so i have a habit of double checking if i read hd somewere. For me this selling of 720 feels like a scam but it‘s pobably my false undersandig.
Apart from that i think 720 x 1440 is okay. 282.45 PPI is okay less than retina iphone (4/5/6/7/8 non plus) but 1080p on 5.7 woud be 423.68 PPI which is unnecessary high in my popinion. The type of display(differns IPS and OLED techhs) matters much more. Nice colores, good brightness and angels stability are really more important on phones in my opinion, as there viewing consitions are so different.
An other point might be the battery consumption. Screens take a big part of the power consumtion and more resultion generally means higher power consumtion. Depenting on the technologie the screen it self consumes more power and also the gpu has to do more work higher res. So this might also be a dession made on power consumtion .