The article doesn’t seem to mention it, but I would assume that a lot of immigration happening during the Viking period was actually against the “immigrants’” will, i.e., the Vikings took captured enemies back home with them to work as slaves? Just a thought…
"A new study based on 297 ancient Scandinavian genomes analysed together with the genomic data of 16,638 present day Scandinavians resolve the complex relations between geography, ancestry, and gene flow in Scandinavia -- encompassing the Roman Age, the Viking Age and later periods. A surprising increase of variation during the Viking period indicates that gene flow into Scandinavia was especially intense during this period."