I don’t recall any viltrued spaceships ever being shown, and Nolan escapes from Earth by simply flying away from it. Allen also arrives on Earth by flying through space.
But wouldn’t it take years if not entire millennia to travel around the galaxy like that? Even going at the speed of light (and that is a generous assumption) it would take them four years just to Proxima Centauri, the closest star to the Sun. To travel the Milky Way from one end to the other would take 100,000 years.
Perhaps Invincible’s galaxy is much smaller than ours?
The handbook mentions that while they can fly at very high speeds, they can also wormhole jump around for faster and further travel. It’s one of the things their smart atoms allows them to do.