The thing with adding to the PokéDex post-game is that there isn't really a whole lot of places for them to be added to. Almost every area can be accessed before the game is over. Unless they expand the Battle Tower (Frontier) to be a full-blown area like in Sinnoh, or they add a whole new mini region like the Sevii Islands, there is no place for gen 4, 5, and 6 Pokémon to be added.
The FRLG post-game had gen 2 and 3 Pokémon in the Sevii Islands. The HGSS "post-game" had gen 3 and 4 Pokémon in Kanto. The only time those Pokémon would appear in Johto would be due to the radio programs--they didn't just change the Pokémon that appeared in Johto once you beat the Champion. So if we're going by those examples, where would the gen 4, 5, and 6 Pokémon go in the Hoenn post-game? The Hoenn post-game was the Battle Tower/Frontier; it didn't have any "new locations" to explore, technically.
Granted, they could easily add an area similar to the Sevii Islands, but then it wouldn't really be considered "Hoenn," but rather a "sub-region" or "mini region," and the scan (supposedly, because I can't make out a great deal of what it says) specifically mentions Hoenn.
It's also unlikely that they removed Pokémon from the Hoenn Dex. Changed exclusives around is a slight maybe, mainly due to Mawile and Sableye being version exclusives, but Mawile getting its Mega in XY. But even then, I wouldn't count on that being what the scan meant.