From the site I gave you you should have found two entries for Epinetus, as Sgt. and Pvt. "Van Scoonhoven Taylor."
These entries mean the Regiment commander was Van Scoonhoven and the Captain was Taylor.
Doing another search for Van Scoonhoven the only commander was Col. Jacobus Van Scoonhoven (also spelt Van Schoonhoven, and often the 'Van' was dropped and the name given as "Schoonover").
Doing a search for a Capt. Taylor in Van Scoonhoven's command, the only one is a Capt. Joshua Taylor.
So Epinetus was listed on at least two militia rosters in Capt. Joshua Taylor's Co., Col. Jacobus Van Scoonhoven's Regiment.
Doing an internet search for Van Scoonhoven turns up a listing for Col. Jacobus Van Scoonhoven as heading up the 12th Militia Regiment from the district of Halfmoon and Ballston (presently in Saratoga Co., NY).
These militia rosters usually do not reflect any active duty service.All able-bodied free white males aged roughly 16-60 years (varies by time and place) were supposed to be available for militia duty.The muster rolls were often created when men reported for regular drill and nose-counting, which was not active duty.
However, the certificate you mentioned was probably a certificate for depreciation pay due him, which meant that he had at least some period of active duty service, which might have been a day or months or more.This makes a lot more sense for a 1781 date than the idea that it had to do with a pension.