In the vein that I am employing appeal to ignorance, you are employing correlation proves causation.
That seems like a bit of a leap to argue, IMO. There's no correlation in the argument, being the biggest hole in your statement.
However, two things are true here:
1) Without direct evidence, we can only speculate. And we have no direct evidence of either claim.
2) We do have a decent amount of historical evidence on the behaviors of Apple and Samsung here that we can use to educate our speculation.
We know that Apple historically has bought panels, not designed them. We also know that Apple has a very good color management system in ColorSync, and has worked on custom drivers for displays like the iMac 5K. We also know that Samsung designs and manufactures this style of OLED display, and has for a while. They are not new at this, and it wouldn't be hard for them to provide the panel used without Apple's input.
From the TV realm, it's also known that you can have 3 different TV makers use the exact same OLED panels and wind up with different results in terms of picture quality, and color accuracy. Color management is important for getting good color accuracy, for example. So more than just the panel itself determines the final result. And displays for TVs, computer monitors and smartphones are similar enough that it makes for a good analogue here.
From that information, we can apply Occam's Razor if we like: The simplest answer that can explain the available evidence is most likely the correct one. One way to measure complexity is the number of assumptions we must make in addition to the evidence we have in order for a claim or explanation to be true. So let's do that.
Your claim that Apple designed it requires
one assumption: That Apple is now designing display panels, something they haven't done before, and we have no evidence for.
The claim that this is Samsung's panel, made to order, requires
zero assumptions. The evidence based on what we know of both Samsung and Apple, along with what we see in the 2017 LG OLED panels that power 3 different brands of TV, is enough to explain how the iPhone X's OLED can beat the OLED in the Samsung S8+, in terms of DisplayMate's metrics.
That makes the claim that Apple designed their own OLED panels harder to prove. It requires that the assumption be satisfied by evidence. That Apple is just using Samsung's OLED as-is, is just simply more likely an explanation because it requires no assumptions to satisfy the evidence.