It makes no sense to me. If I had to guess, I would say that it means something like "over-excited words and phrases" or "extraneous information" suggesting that the writer had got rather carried away with their writing and needed the editor to bring it back down to a sensible and clear version (as shown by the fact that the writer is thanking the editor for giving a "clear, precise sense of this story"). Perhaps the editor got rid of the "waffle" and the "flannel" from the writing, tidied it up and made it more understandable. That is, after all, an editor's job.
I can't find a definition of "wahoo" that fits that, though. Had it said "yahoo" (without a capital letter) I could have connected it to the exclamation of delight and wonder which people sometimes make and which, I suppose, could be used to describe over-excitement in a piece of writing, but I've never heard that word used that way either.
Student or Learner