They're reasonable extrapolations from current capabilities. Build a self-replicating 3-D printer, make it capable of finding and refining raw materials on asteroids, slap it on a Voyager-like slow interstellar platform, and off you go.

(Yes, I'm being glib, but it's not that much of a stretch.)

Yeah but the energy efficiency required to pull that off? That's not just futuristic, that's astronomically close to violating physical laws...

"reasonable extrapolations" is a far cry from "airtight logical argument". But these discussions rarely seem to go anywhere...