Although a Nobel Prize winning author, Knut Hanson was, to put it mildly, pro-German, to the point of being openly pro Nazi during World War II. His views about the British blockade in World War I, or other topics involving Germany, must be viewed in that light. The fact of the matter is that the British blockade did cause a certain amount of starvation in Germany, and he may have conflated that with "starvation in Norway."
As an author, he was also against "realism" and "naturalism," preferring to believe in a "mystic" past. Born in 1859 (but living to 1952, that is, to his nineties), Hamsun was no longer a "young" man during World War I--except possibly in his own mind.
So it's quite believable that he said the quote attributed to him. The truth of the alleged statements is open to the judgment of others.