Maybe it has to do with his hack on the ROM?The Mega Lati@s we saw didn't have the mega symbol that other megas have over the "Lv" symbol. It's likely either fake or a placeholder. Plus the duo would need Dragon/Fairy and/or a 200 BST gain to be worth a look over Soul Dew Lati@s.
I won't even begin to understand how he was able to do this, my assumption is he probably changed the wild pokemon encounters to a set pokemon ID number?
Again assuming, the only times a pokemon go mega in the game are when a trainer commands the pokemon to do so. So by following my previous assumption, he would have to change the encounter to BE the pokemon already mega'd?
I don't know, not going to write it off as fake right away.