I believe that the anon over on TTDDTTOT#2 was led astray regarding the registration of the Dash-8 plane. Kiribati regs do have a T3 prefix but are followed by three letters T3-AAA through to T3-ZZZ so T3902 does not resolve to a Kiribati aircraft.
With regard to the wiki listing for Air Kiribati stating - "It also operates charters, medical evacuation and search and rescue services." - this is not surprising for a group of islands approximately 1000 miles south of Hawaii.
Kiribati is a small country with no need of an air force but yachtsman/shipping can be in distress or people sick/injured on remote island in the group and planes are the only viable rapid response option. Therefore it is not surprising that the national airline carries out the duties of an air force or coast guard.
The Twin Otter & Harbin Y-12 are broadly similar aircraft that are ideal for short range island hopping landing in grass or beach/sand strips, they lack the range to venture far beyond Kiribati.
The Dash-8 has a longer range to fly to nearby neighbouring island states.