Superman may not be able to time travel under his own power (and hopefully he never will), so Superman using the Flash's Cosmic Treadmill (for the first time in history that I know of) to time travel is a pretty awesome feat. I didn't know Superman could do that.
He couldn't. The concept logic behind the Treadmill was that although Barry Allen could time-travel under his own power the Treadmill refined the process and allowed for him a greater accuracy and control - the actual mechanism that allowed Barry to travel through time and space and always find his way back was his mastery over his own molecular structure and vibrationary rate. He could sense and attune his own vibratory rate to where he needed to be, and Superman could do something similar himself, though his was more an intuitive exercise than Barry's more fundamental sensitivity to such travel and pinpointing specific destinations - it was why The Anti-Monitor saw him as such a potential threat and targeted him early in the Crisis.
In today's continuity The Flash supposedly cannot exceed the speed of light, so whether the Treadmill is supposed to still run off his own vibratory powers and that that is the secret of his current time-travelling is a little unclear. But logically it must be so - the Treadmill in itself is not a time-machine! It is purely an accessory, an aid.
Botht the Flash and Superman have been shown exceeding speed of light though in recent continuity!