I also see that your same "scientific" source clearly states:

Region of northern Europe, usually defined as comprising Norway, Sweden, and Denmark. It is sometimes used more broadly to include Finland and Iceland. Norway and Sweden occupy the Scandinavian Peninsula, though Denmark is part of the North European Plain. The Scandinavian peoples are linked by cultural similarities, and they speak a closely related group of Germanic languages.

Scandinavia is a region, rather than a geographic peninsula.