This makes sense right up to the part where it assumes that 8-10 years pass in between the first episode and Call of the Cutie, where Apple Bloom is clearly in the same grade in school that she is currently. I doubt that the first 12 episodes take place over the course of a decade.

A much more likely explanation is that '100 moons' means something entirely different in a world where the moon is manually raised and lowered every night.

… Or maybe the 3rd Reunion was not a formal Apple family reunion, but they just came to Ponyville to attend the 1000th summer day with Princess Celestia…