If I remember correctly, Jinora mentioned to Kai that astral projection is a high level of airbending with a little spiritutality involved and it was easier to perform during Harmonic Convergence. When she said the "high level" part, then to me that sounds like it's a sub-skill to airbending as much as bloodbending is to waterbending and lightning-bending is to fire. To me, the way that Jinora described it makes it sound like the other sub-skills. And the fact that only Jinora has been able to perform the skill appeals to me as a sub-skill as well. All the other sub-skills are high levels of bending like astral projection is. Astral projection has spirituality involved, just like airbending. See the connection? And the way that astral projection is easier to perform during a time of high spirituality makes it sound other sub-skills, like how bloodbending is easier to perform during a full moon.
In my opinion, astral projection fits the description of a sub-skill for reasons listed above and it deserves a place in the Airbending page on this wiki (it looks like it already has a place on the Specialized bending techniques page). I made the poll to see what others would say regarding this subject.