It's really confusing to use the term pacific islanders here. The farthest humans had reached was the Solomon Islands, what we call "near Oceania". The places people think of when they say "Pacific Islands", namely Remote Oceania (Melanesia), Micronesia, and Polynesia were all uninhabited. Those early Papuans weren't especially closely related to the founding populations of the Americas, though they're closer than you might expect from the geographic distance because of the serial founder effect.
Yes, that is a really extreme simplification. There were at least three major groups of early humans around Southeast Asia and the Pacific Islands 20k+ years ago. The main point of interest is that all of these groups were genetically differentiated from the Siberians who migrated along the vicinity of the Alaskan coast later and who left a genetic trace on native populations that is still observable today.
