My guess would be that they do PWM it, but I don't know that for a fact. The problem is that changing the current through an LED can also change its color output-- given how hard Apple has tried to keep their TrueTone color accurate, I'm guessing they are precisely controlling the operating current.
There's also problems with loss of efficiency and a minimum on current (thus a minimum brightness as well).
The PWM allows them to keep it on at exactly the desired current, and then off entirely-- so the color of the output is controlled by the on current and the brightness by the ratio of on time to off time. Controlling the duty cycle finely enough also allows backlight brightness less than you'd get by running constant current at the lowest level.
I haven't noticed any flicker, but it could be that I'm just not sensitive to such things, or it could be that Apple's PWMs are at a high enough frequency that I can't notice.