It's not flicker, it's the repainting of the image faster. Smoother motion.
If superman flies across the screen in 200 frames instead of 60, then he's got a smoother transition between each still image.
But the way persistence of vision works, you detect brightness quickly, but our eyes' sensors can't register things going dark quickly. (Think camera flashes - they can be well under 1/1000 of a second. You sense them easily, but have no idea how long they lasted.) It's to do with the bleaching effect.
This means, for example, you could just paint superman on each of those 60 frames 3-ish times and our eyes would see the same thing as if you showed 180 frames in the same timespan.
(A football travelling across a pitch is a better example as it's white moving over relatively dark green.)
Besides, Superman has slowed down a lot since he hit the bottle.
