Dunmore's Proclamation didn't apply to New York. Dunmore was the royal governor of the Virginia colony, not New York.
Hewlett observes that "slavery is a sin" and claims that it has already been abolished in England. While there were anti-slavery sentiments among the English population, and a court ruling in 1706 had declared that enslaved Africans brought to England would be considered free, slavery itself would not be officially abolished by Parliament until the Slavery Abolition Act of 1834.