It means you don’t understand English. “Dedicated to the proposition”
So you are saying our "New Government" in 1776 was "dedicated to the proposition" that all their slaves should go free?
So why didn't they do that? The history doesn't show them to be dedicated to that proposition at all. It shows them to be dedicated to the proposition that they should be free of England, and able to govern themselves, much as the Southern states tried to do.
They kept their slaves. They didn't free them. It was years later before the first states even started trying to free them.