I'm not really buying that.
I'm certain an OLED panel costs more just like in TVs but you know Apple wanted a significant price differential between the 8+ and X. They set it up at $200 and I do not see that Apple would release the phones with only a $100 differential. Why would anyone chose the 8+ in that scenario?
Look at their current pricing structure of the highest storage models of their lineup. It's around $200 spacing between each model. If anything it's Apple's pricing of the lineup dictating the high price tag, not simply the display. If it is the display then they're severely overpricing every other model.
X: $1,149
8+: $949
7+: $769
6s+: $649
SE: $449