I'm guessing "Post-game Content" to some reviewers meant something along the lines of raising Pokemon, battling others online, participating in contests. Pretty much things you could do even before battling Steven. The only difference being that these activities are a lot easier once you become the champion. I think the Delta Episode/Rayquaza, the Regis, and the mirage spots are the only real pieces of post game content since the Lati twins are going to be obtainable beforehand.
The Battle Frontier is assumed to be gone according to the usgamer review, but we have to be getting something. Whenever a new game is introduced within the current generation, the post-game battle facility is largely improved over the recent one, or at least it has been since Hoenn. The term "Resort" hints at there being some sort of park with several battle facilities, but a revamp of the original Battle Tower seems likely given the reviews. I'm still holding onto some doubts about it though.
However, I am left wondering why and how could they do this. Having another Battle Maison/Tower/Subway right after dealing with one from the previously released game is a step down from what players would expect. You could only go so far with the current content they have revealed for the post-game. While I'm not saying that I'm not grateful for what we are getting after beating Steven, there is not much you can do besides raising Pokemon for monotonous style battles. The day 6v6 singles becomes a default option on Battle Spot, I will shut up about what I just said.
Catching legendaries through the mirage spots, done. I've filled the Pokedex bar Events and now the GTS has a legit reason to be flooded with even more Legendaries (try to find the ones that are genned). I feel as if nothing is special anymore. I can imagine showing a friend a Palkia that I have brought over through my original Pearl while he shoots me back with 6 Giratinas, 1 from a mirage spot and 5 which were obtained through the GTS.
Regarding the Delta Episode, it appears to just be another main post-game story that will only last a half hour. Not that this is a bad thing. A meteor is crashing into the earth and we have to ride Rayquaza into Space in order to stop Deoxys, all the while learning the origins of Mega Evolution? Hell Yeah! This is a much better post-game story than what we had to deal with in XY, the boring Looker mission. But as I have mentioned before, it will not last long and the only rewards we could get would possibly be a Mega Stone, Rayquaza/Deoxys, and Zinnia as a repeatable trainer to battle against. Also, did anyone feel insulted when the trailer for the Delta Episode asks what the Delta Episode meant for the fate of Hoenn? Spoiler Alert ***Hoenn doesn't blow up and Team Magma/Aqua fail again!*** If only the games allowed tragedy or to have the villains win for once.
Anyway, the games seem to have the objective of either raising Pokemon, collecting Dex entries, and "being the best like no one ever was". The whole concept revolves around grinding, something to keep the player busy for hours. I believe that it has a strong fan base because it has both a casual and competitive nature, qualities that appeal to all types of players. So yes, I am confused about why the Battle Frontier is supposedly deconfirmed. But we can't be 100% sure that we will get a full 'cut and paste' copy of the Battle Tower until the games are actually released, or when the leakers decide to finish the game before then. So I'm keeping my fingers crossed, maybe Game Freak has a few surprises left for us.