I once worked for a 121/135 operation. The feds signed off on 121 crews flying the 135 airplanes so long as the airplane was operated by the crew to the 121 standard and operated (ie. revenue/seats/flight following)/maintained by the company to the 135 standard.

---WARNING, THE FOLLOWING MAY BE OBVIOUS TO SOME---

My guess is GLAs 135 certificate is, as far as procedure go, identical to the 121 certificate and is being taken advantage of to (in a rather clever way I might add) opt out of the 1500 hour requirement.

If they were smarter (and even more clever) than they are they would have gotten a Combi conversion to 9 seats (very common in AK) and utilized the new cargo space to obtain revenue from FDX, UPS or a freight forwarder. Particularly as they rarely see more than 9 passengers to begin with.