That's actually addressed in the missing story, "The Traders", when a couple of these asyouknowBobs from the Foundation are talking about the closed worlds of Askone.

"That so? But they have nice little ships which spotted me very handily two parsecs away. That smells of nucleics to me."

Gorov shrugged. "Those ships are holdovers of the Empire, no doubt. Probably with nuclear drive. What they have, they keep. The point is that they will not innovate and their internal economy is entirely non-nuclear. That is what we must change."

So the absence of knowledge about nucleics (magic fairy dust) does not exclude the presence of nucleic technology in the "barbarian" worlds.