In 1931, Newfoundland was a separate dominion, not part of Canada. Because of their size (smallest of the six dominons) they relied heavily on Britain and weren’t keen on being pushed out of the imperial nest, so didn’t want the Statute to apply automatically; wanted to review it locally and decide if they wanted it to apply to them.
Then two years later Newfoundland went bankrupt in the Great Depression and had to surrender their self-governance in return for British financial aid.
They were under British governance until they joined Canada in 1949, when the Statute then applied to them as a province.
This is mentioned generally in the wikipedia article on Smuts, but not the specific issue of the name:
While in Britain for an Imperial Conference in June 1921, Smuts went to Ireland and met Éamon de Valera to help broker an armistice and peace deal between the warring British and Irish nationalists. Smuts attempted to sell the concept of Ireland receiving Dominion status similar to that of Australia and South Africa.[48]