I think it has video playback of 1080p but like on 5 inch display. Nowhere near real 2k...
4k is not supported by the screen.
I had gionee elife e7 with 1080p screen 5.5 inch back in 2013 (same is like on 6,7,8 plus) and then lg g3 with real 2k 5.5 inch screen and there is a huge difference.
Also I had 5, 5s and 6, but I think I will go with Sony true 4k hdr screen.
Thx for your answers!