The Militia was all able-bodied free white males roughly aged 16 to 60 (varied by time and place) living in a given County.
County militias' purpose was defense within the County when needed.It was extremely rare for militia units to be sent outside the County.The exceptions were usually on the frontiers and in event of major disasters, such as the siege of Charleston.
Lancaster County was not a frontier area, but Tory/Indian raids or depredations, or rumors of them, were not impossible.
The peace treaty was not signed until mid-1783.
Active duty for militiamen could have included patrolling, assisting to escort supplies bound elsewhere, or guarding prisoners.
Muster rolls for militia were usually drawn up when men reported for regular drill and nose-counting, which was not considered active duty.A lot of these rolls have not survived.Some may be among County records or library or historical society manuscript collections rather than in the PA Archives and may not have been published.
It is possible that your ggggggrandfather had some active duty service but that the paperwork has not survived.
When you refer to militia matters, ~always~ give the County ID, much more important than a battalion number.