Even incandescent running on AC has flicker. It's funny when that's used as the gold standard. LED running on DC has less flicker than normal incandescent.
An LED running on continuous DC has no flicker, but PWM is not continuous DC, it's a square wave of some frequency.
Incandescents have analog inertia in the filament which smooths the light output from the AC sine wave. This smoothing is not 100%, but I've never met anyone who can detect it without equipment.
A photocell and an oscilloscope will show the smoothed live-frequency wave (I wouldn't call it a "flicker"). The wave delta is relatively higher in the perceptual range as the voltage is lowered to approach the minimum "glow-activation" threshold of the filament -- i.e. the fluctuation is more noticeable when the bulb is dimmed to nearly off.
