By ‘northern Germania’, the archeologists mean ... Denmark.
Germania is an exonym that Romans invented. Indeed, the ‘Germanii’ tribe is probably a Celtic tribe. Most of Germany (Latin Germania) is Celtic tribes, especially from the previous Celtic La Tene culture that covered most of Germany.
Notice where the socalled ‘Proto-Germanic’ originated! Norway, Sweden, and Denmark! It originated in Scandinavia. Socalled Proto-Germanic is a Nordic parent language that is indigenous to Scandinavia that developed from within Scandinavia.
Notice there are two separate colors. The red is the Nordic cultural Zone. The magenta is the ‘Germanic’ cultural zone, associating archeologically with Jastorf culture. It is uncertain if the Jastorf ‘Germanic’ culture is actually speaking any ‘Germanic’ language at this time. Perhaps, they might have adopted the Nordic language from their northern Nordic neighbors. Or the ‘Germanic’ culture might be speaking a Celtic language from their neighbors.
Notice which direction the Nordic (‘Germanic’) language spreads, from Scandinavia into Continental Europe.
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Continental Europeans adopted a Scandinavian language for unclear reasons that appears to radiate out of Denmark.
When these ‘Germanic’ tribes adopted the Nordic language, they also imported, borrowed, and reinterpreted indigenous Nordic traditions.
Scandinavia is the origin of the wrongly named ‘Germanic’ language. But every Nonnordic culture that adopted this language, no longer had the original Scandinavian context. In the new Nonnordic contexts these Nonnordic cultures completely reused and reinterpreted things like Nordic sacred names, according to local Nonnordic traditions.