Your model probably does use PWM. Later models reportedly use it in the following way:
"I have a friend who works in R&D for Apple in California and he got me in contact with the head of displays for Apple. The jist of it is that 2015 MacBook Pro 15inch use no PWM until 50% brightness, at which point they use a 100Khz PWM frequency, which is well above the migraine threshold and is "undetectable" to most tests (even some cheap oscilloscopes)"
https://ledstrain.org/d/6-laptops-and-you