Is there a reason they couldn't just jack up the PWM frequency? Most OLEDs seem to be under 500 Hz[0], which leads me to believe that there's something limiting them from performing any faster. Basic LEDs on the other hand can easily operate at frequencies in the 1-5+ kHz range and can be pushed very far if the entire system is designed well. A display running at 10 kHz might not be noticeable to even the most sensitive people.

[0]: https://www.notebookcheck.net/PWM-Ranking-Notebooks-Smartpho...