This is a mischaracterization in the same class as what's described in the article. Early Rome was just as tribal as the rest of Europe, and Greece has been tribal for almost its entire (ancient) existence. The major difference is that the tribes of Northern Europe seem to have been much less well documented, but they didn't live in isolation. There's ample evidence of trade routes spanning from the Iberian peninsula to the Carpathians, and across the British isles, the Nordic and Baltic states.

The "bunch of tribes" encountered by the Romans was actually a vast Celtic empire, which was itself preceded by other societies such as the Bell Beaker or Corded Ware cultures. But they've largely disappeared from the record because of the written legacy from the Greek and Roman empires.