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Isolationism as expressed in the base note is clearly false. Here is the single counter example required to refute it: the Clovis culture in North and South America. The Clovis tools and spearheads are found in multiple sites over a huge geographical area, within a relatively narrow range of dates. There are more than one thousand known sites with Clovis spear points or toolkits throughout both North and South American.
Key thing is the dated range of all these sites is about 500 years. So there is an example of a technology that diffused pretty fast across two large continents.
Note that disproving isolationism does not prove diffusionism in general. But for this one technology, diffusionism is clearly how it spread so fast.