It depends on what happened to the license when the merge happened. Did the author of wahoo transfer the license to the authors of oh-my-fish? If so, then the code at the time of the merge is not subject to this DMCA notice. However, as I brought up, if after they had this tussle and split up, and oh-my-fish kept pulling new code from wahoo, then that code would be under wahoo's license and would need to be attributed as such (and I don't think it was attributed, hence this takedown request).
My point is that if they agreed to merge projects and wahoo didn't keep their license on it, then that code at the time of agreement is no longer under the exclusive copyright of the wahoo author.