OpenGL is part of the platform, they all support it. The stats page doesn't even include 'not supported' [1]
Being able to run anything slightly demanding is other thing, but you can't argue there's no support.
Also, the benchmark you linked is for application load, which is heavily influenced by storage speed and load method (android has to JIT compile sometimes) and has almost no impact from the graphics' performance other than the bus between CPU/memory and GPU
Being able to run something suboptimally doesn't turn into sales. I'm sure that the owner of a $70 Blu R1 HD is not going to be spending money on high end games.
Also looking at Apple's sells every year since the 6 came out, I doubt very seriously that Apple has sold more 6 phones than 6S, 7, 8, and X phones.
Also if the 6 from 2015 is not a powerhouse, neither is Samsung S8 that was just introduced last year....
http://bgr.com/2017/05/23/iphone-6s-vs-galaxy-s8-speed-test-...