The headline is wrong. So is the conclusion drawn by the author. Funnily enough, this isn't the first time MR has made the same mistake. DisplayMate is
definitely not saying the 12PM has the best smartphone display it's ever tested.
It is saying the 12PM has
qualified for and received the A+ rating, which is it's highest performance grade.
The linked DisplayMate article explains this pretty plainly.
Relevant excerpts:
And to get a DisplayMate A+ Display Performance Rating the Average Absolute Color Accuracy and the Average Absolute Color Accuracy Shifts with Average Picture Level APL must All be less than 1.0 JNCD, and the Largest Color Errors must All be less than 3.0 JNCD
As a Result the iPhone 12 Pro Max display meets all of the criteria and requirements for a DisplayMate A+ Grade, earning DisplayMate’s Highest Overall Display Assessment Rating and Highest Display Performance Grade of A+.
The iPhone 12 Pro Max joins the very select Top Tier of Smartphone Displays which all provide Close to Text Book Perfect Calibration Accuracy and Performance that is Visually Indistinguishable From Perfect, so they all received and maintain Concurrent DisplayMate Best Smartphone Display Awards. All are Excellent State-of-the-Art Displays, each is better in some Display Performance Categories, but None are Best in All the Display Performance Categories. Note that measured numerical display performance differences that are Visually Indistinguishable are equivalent.
tl;dr The iPhone 12 Pro Max received the highest rating that DisplayMate gives: A+