People are over-reacting. As many people have stated already, nothing in this issue says that Emerald Twilight never happened and in fact several previous issues confirms that it did happen (at least in some form).
All this issue proves is that 2 years ago, Kyle goes to Oa for the first time and gets 'official' training. It could easily take place after the Corps is resurrected, since before that he would have no reason to go to Oa, and he would have been relying on 'un-official' training that Ganthet provided.
If we are to look at just the facts, as provided by only New-52 GL issues, there is way more evidence saying Emerald Twilight DID happen, compared to the contrary.