While I think that is an interesting theory, I personally believe that a far more plausible reason for the game being severely lacking is that simply Nintendo was behind on schedule. They already had that nine month delay, and heaven knows that if there was another delay, the fans would book the next available flight to Japan and set fire to the Nintendo development offices. So in order to make things work, I believe that they had to cut all the (many of them basic) features out and give vague promises about future updates (which we really know nothing about) so that the fans wouldn't question the severe lack of content in the base game they paid a whopping $60 for.
If the time travelling theory is true, why would they allow for the guidebook to be released? Wouldn't that just defeat the purpose of keeping all these features a "secret"? People could just look in the guidebook and leak everything that way, anyways. Personally, I'm hoping the guidebook gives us a glimpse of what's to come.
This. It's obvious the game has been behind schedule since it's announcement. Someone from the higher-ups in Nintendo probably told the dev team they needed to be out by the end of Q1 to prevent overlap with other releases, so they pushed it back as much as they could.
I do think we'll get updates in the future with more than just holiday stuff. However, I agree we shouldn't assume what is coming and what isn't. Are we getting art? more Nook's Cranny upgrades? gyroids? Most likely, but until Nintendo says something, it isn't official.
That being said, I do really enjoy this game. The real negative for me is glitches, which probably has to do with trying to get the game out quick. At least Nintendo seems to be working quickly to fix these. However, I'm really enjoying this game so far. I feel the features added outweigh those removed (in a perfect world we'd have both, yes).