I subscribed to OAG for quite a few years and was a regular reader of Frequent Flyer magazine, which is a giveaway that comes with the OAG flight guide. I'm pretty sure that the letter writer was complaining about a string of letters in previous issues of FF that contained the various whines referred to in the letter. Note that "Forum" is the name of the letters section of FF.

It struck me as kind of ironic to have a whiner whining about whining!

FWIW, I dropped my subscription to OAG this year, got a subscription to InsideFlyer after reading a few sample issues I somehow got ahold of, and took out a subscription to the AmEx SkyGuide that came with a nice bonus of 5,000 SkyMiles.