It was known that carbs cause the problems centuries ago and should be avoided, but dairy and fats should be eaten generously. Low carb/high fat diets was the main, if not only, treatment for diabetics (they weren't split into types then). Even after insulin was invented this was the case.
Cookery and Household Management
Printed 1936
Diabetes
Those suffering from this ailment require carefully to avoid all foods containing sugar and starch. The following Must be avoided:
Milk, sugar, flour, cornflour, oatmeal, rice, sago, macaroni, the various pulse foods, fruits containing a high percentage of sugar, potatoes, beets, carrots, peas, parsnips, broad beans, spanish onions.
The following are allowed:
meat, soups, fish, poultry, game and meat of all kinds. Also eggs, butter, cream, cheese, certain vegetables. Light dry wines. Weak unsweetened spirits. Tea, coffee and cocoa which may be sweetened with saccharine. There may be plentiful use of butter, cream, fat and oils if the digestion will allow.