I assume your response is coming from a low-carb/paleo perspective? No leafy greens or vitamin-rich vegetables. The vegetables that are there (root vegetables) are carby and not especially nutritious. Huge vitamin deficiency across the board.

Not when you eat the entire animal including organ meat as these folks undoubtedly did.

That is simply incorrect. For example, beef liver is about twenty times more nutritious than fruit by mass.

My original question was a bit cheeky as I already knew the answer.

Yes, but don't forget that for a relatively small amount of a beef liver you need to kill a whole cow. And when you kill the animal you couldn't cherry-pick the parts of the meat you use, everything was used while it lasted. The parts that could be preserved would last for a while and things like a liver would be eaten right away or used to make sausages. In the middle age common folks didn't eat meat as often as we do today, it was a luxury. They primarily used a lot of buttermilk, cheese and eggs instead.

You get your diet from a warm supermarket. Or heck, delivered to your door.

The Icelandic ancestors didn't have that luxury in their country's bitter cold

It's not the same cooking you've come to expect. There is a whole wikipedia entry on the subject: https://en.wikipedia.org/wiki/Icelandic_cuisine#Medieval_Ice...