There are a couple different types of diffusionism. The standard meaning refers to the old theory that new ideas are rarely invented, often borrowed. Things like Agriculture or ironworking were only discovered once (or few times) and between cultures. This is pretty characteristic of archaeology from the early 20th century, but it has some significant weaknesses as a theory. It's been superseded by more modern theories, but you can still see bits of its influence scattered about.

Hyperdiffusionism is a more extreme version of diffusionism that basically says everything was invented exactly once by some ancient precursor culture and spread to the rest of the world. This idea is understandably popular among afrocentrists and similar groups.

Both types would get you laughed out of any archaeology department today because they have glaringly obvious flaws an expert should be able to notice. But you seem to be asking about the precolumbian transpacific contact, especially the stuff associated with Heyerdahl. Thor does come from a diffusionist school of thought and his Kon-Tiki expedition was designed to support diffusionist theories, but modern archaeologists have found very little evidence to support significant contact between Polynesians and the Americas. This post by /u/Reedstilt is a brief summary of some of the evidence for contact at all. In the north, there also contact across the Bering strait, although of a fairly limited nature as well. Outside of these areas, there is no reasonably debatable evidence of contact until the Viking expeditions at l'anse aux meadows and the later Columbian expeditions.