The Gauls, Britons and Insular Celts (Ireland and Scotland) were all Celtic, as well as many Iberians (Celtiberian). And, of course, the Celtic tribes were dominate throughout Europe before the time of that video, and then the Germanic tribes came to dominate. And even then, such as with the Germanic tribes the Teutons and Jutes and Danes would not have considered themselves the same even though today we lump them together.
And then, of course, it is strictly by what some, such as Rome, claimed. Lots of European tribes had a very different idea of that.
And you can't really settle anything territory debates with it with just showing such things, such as the history of Poland. Or how the Saxons really don't appear to have invaded Briton but where rather invited as mercenaries.