He time travelled through time during the GDS. That's how he gets to the 30th century. Darkseid was amped off the Time Trapper, so he should have the ability to prevent Thor from Time Travel.
That's crazy. How did they defeat DS during that event anyway?
Ultimately, it goes like this:
The Legion of Super heroes attacks, he beats them. Highfather and Orion attack him, he beats them. Supergirl & Superboy attack him, he beats them. He is forced to retreat when 3 billion daxamites attack.