Rex went back, way back.

Having watched the Xentash calamity play out in reverse, Rex realised time was as malleable to him as his own form.

He knew interfering with the Xentash would alert Jones to his own ability to bend time, and at that point Jones would never be stopped.

But maybe if he went back to a time before Jones had the Ortrix, he could find a way to unravel it all.