Yes it is true, but overpopulated does not necessarily mean that the place was overflowing with people, it just means that there were too many people relative to what Scandinavia could support. At the time Scandinavia was a tough place to live, it was cold and hard to grow crops there compared to Britain and France, and most of the land couldn't be settled (Norway was too hilly, Denmark was quite small and didn't have much arable land, Sweden was covered in dense forests). Actual liveable land became scarce and people wanted to expand outward. There are undoubtedly way more people living in Scandinavia today, it's just that developments in agricultural techniques that came after the Viking Age made it easier to live there among other things.

I would not call it over-population, but there was a surplus population.

The early medieval warm period had led to larger harvests and better chances for survival for children - these children would either have to become brytar, people going off into the woods to burn forest and create new farmland or try to go somewhere else to gain their own land. Both Norway and Denmark became centralised kingdoms during the era, and the royals attempted to centralise rule and acquire more land to fund their dynasty. This led to conflicts with once-powerful local families, some of which decided that taking their chances in Iceland or Greenland, or go to try to carve out their own petty kingdom in Normandy, Ireland, England or Russia was a better idea than to chafe under the king, so they moved, or raided.

There was thus a surplus of population that were willing and able to move themselves to other countries to colonise and raid (or trade), rather than more population than the country could feed.