If that was the case, then why weren't Hoopa, Diancie and Volcanion protected by the same measures? That's the crux of my argument: I understand that they could add moves by patching, or they could have all the moves in the games from the start, but they would have no reason whatsoever to half-ass both solutions at the same time. Why include some (note: still enough for a generation's worth of events) content in XY, then plan to add more as patches later? If you've got the means to add content to the first games, why not include it all in the first place? Likewise, if you have plans to add stuff through patches, why include extra content in XY at all?
Logistics-wise, pre-loading it all into XY would be the easiest thing, as there would be no need to issue patches. This is also the solution they've gone for for three generations straight.
Secrecy-wise, patches would ensure no leak of events beforehand, but if that was the motivation, they've ruined a lot of surprises by keeping a solid portion of the event Pokémon and moves in the games at launch.
You could argue that the content hidden in XY was a bunch of "red herrings", but it seems far too substantial for that. Had there been only one or two moves, and a few unused items, I might have been convinced, but XY had far too much stuff hidden on them for a simple distraction.
There's also the possibility that they will drop compatibility altogether, but that seems way too unlikely to be the case. Whatever tricks the new Groudon/Kyogre formes have up their sleeves, it would be in GameFreak's best interest to make them able to showcase their abilities in battle against XY cartridges. It's a part of their marketing strategy after all.