The fact that Britain and Scandinavia were, in ancient and early Medieval times, actually very diverse places, is interesting, because within living memory those places are /believed/ to have been > 90% "white".
This raises a number of possibilities for where those beliefs come from:
Option 1: It really is true that, around 1950, those places were > 90% "white", but in ancient times they did not used to be. This would mean that the diversity present in ancient times had subsequently been forced out, either via systematic oppression that motivated out-migration, or via active genocide.
Option 2: Census and other data from the past century were systematically falsified, consistently and at a large scale, to support the regime of white supremacy that then dominated Europe, when in fact those populations were considerably more diverse; non-"white" people were just "officially invisible". Moreover, the commonly-reported subjective memories of older people who say they remember such a time are in fact misimpressions -- because memory, as we know, is unreliable, strongly affected by whatever the social reality is.
In other words, either
- large-scale population replacement occurred in these places within just the recent past, or
- the past is tremendously mutable, in which case nothing you understand as history, nor indeed anything you think you remember, can actually be trusted.
Either conclusion would be very compatible with your opening point that everything is really politics.