Supposedly, his problem was with some production people when he was on DW. I believe almost all of the production team has turned over, at least once. He also stated his favorite episodes were Moffatt penned. He also supposedly met with Moffatt recently to discuss the 50th but declined. So using the "problem with someone in production" is now an invalid defense.
Well the event, where he sent the letter was about the time he was on the show.
Whether to appear in the episode or not, is a separate matter.
Seems like he didn't like what he heard was planned for the episode, unless he went there just to get some spoilers.