They're in interstellar space, using an FTL drive, but the First Contact team who met up with them can't figure out how or why -- by all rights they should be stuck on their homeworld because they've barely figured out atomic power, or steel, or starting fire with flint. They're an anomaly. Sometimes it's because they've stolen a technology they don't know how to create themselves. Sometimes a Precursor race gave it to them or left them instructions on how to build it. Sometimes it's a simple fluke. But whatever the reason, they've made their way into space far earlier than other civilizations might think possible.