The aircraft, manufactured by McDonnell-Douglas which was later bought by Boeing, requires a flight engineer to manage the fuel supply while in operation rather than having a computer monitoring it like on newer aircraft.
If the author (who was not noted, that I saw) meant to imply that a "flight engineer" was a normal crew member and necessary to monitor the fuel supply on the type they could not be more wrong. Dismal "reporting" really.
The DC9 - MD80 is certified as a two pilot crew, not three. If memory serves me correctly the DC9 was the first U.S. transport category jet designed and certified for a 2 pilot crew.
The accident aircraft had a dual channel fuel quantity indicating system that fed real time fuel quantity indications to the instrument display in the cockpit. The zero fuel weight may be preloaded in the instrument as well so the instrument displays real time aircraft weight. I have no knowledge of whether the fuel quantity indicating guage and related system was functional on the accident flight. But to imply that a third crew person is necessary to monitor fuel quantity on the MD80 flight deck is nonsense.