That is based on something John Adams said. He may have been unduly pessimistic. But there were a lot of Tories/Loyalists in some districts which sometimes was the result of local animosities rather than their attitude towards King George III.
We had an old neighbor who was born in Atlantic Canada, who died at age 90 about forty years ago, and years later my Dad said that his family roots were in New England but they left during the revolution, and he had belonged to the United Empire Loyalists. An organization which I had never previously heard of.
The academic support for the 1/3rd scenario comes from Crain Britton’s Anatomy of a Revolution, which was a study of the French Revolution. He based his conclusions on public accounts, diaries and journals, and the notes from the various revolutionary courts. He then postulated that his findings could and should be applied to most revolutionary movements, particularly ours.