Ichabod Crane says that he worked along with Mohawks (after deserting and joining the Washington) and Americans were using them as spies. Although several tribes allied with the rebels, Mohawks were actually allied with the British.
It is possible however, that the writers were thinking of the Mohicans who did side with the colonists during the Revolutionary war. The Mohicans are an Algonquian tribe whereas the Mohawk are Iroquoian.