The letter is actually from Stannis and was meant to motivate John to gather the wildings, arm them and march south to meet an oncoming Ramsay who is ready to slaughter anybody and everybody who stands in his way of getting what he wants. In other words, Stannis tried to dupe John.

I think this might be supported because in the text Theon remembers telling his sister of Abel and the washerwomen and their true motivation. So, Stannis probably knows and has put it together that they are wildings sent. Did Stannis know about Mance still being alive? I can't remember. I don't really think Rayder and his women who seem like unstoppable super-assasins were all caught by the Bastard. I can't remember the end of the book entirely but I don't think it was clear if any were caught was it? I think I just assumed they were caught because of the letter?

Also, if there was a battle and Bolton defeated Stannis then how did he do it? His ace in the hole was the Karstarks turning on him. Also, if there really has been a battle then what of the Manderlys? It seemed pretty obvious to me that once they reach Stannis's camp they are going to turn on the Freys, slaughter them and then either sit still to talk to Stannis about what the Onion Knight is up to or just straight out join him.

I'm not entirely clear about the end of the last book and I don't really want to go back and look again, but it seems to me the whole letter was a ruse on Stannis' part. It makes more sense than trying to paste together timelines to figure out if there really was a battle yet or not.