People believed that the colonists had really been independent prior to the Declaration of Independence because of the practices of salutary neglect.
Salutary neglect was practiced by Great Britain and put onto the colonies. Although the name sounds bad, it was a good type of neglect. Great Britain did not enforce any trade laws or unfair laws onto the colonists, they pretty much let them do their own thing. As long as the colonists were making Great Britain wealthier, they didn't care and left them alone.
This all changed when the French and Indian War ended. Great Britain needed a way to pay for the very expensive war and they were using their troops to protect the colonists. This was when Great Britain got rid of salutary neglect and started to tax the colonists, which the colonists opposed and started the saying, "taxation without representation."
Even after all of this, some people didn't know if the colonists could be independent from Great Britain and thrive on their own, even though they had already been independent before.