Nintendo doesn't have a huge history of releasing demos. And when they do it's usually of games already released. But the wording of the tweet is alluring to say the least.
evidence in favor of a demo is usually that they try demos for games they expect to do well or want publicity for, like smash 3DS, MH4U and codename S.T.E.A.M.
however, all of those games have 2 things in common: being on the 3ds, and being developed by outside studios (sora & namco, capcom, and intelligent systems)
i think nintendo might just do demos on the 3DS because it's their best performing system currently being sold and most of their userbase is on there
i do WANT for a splatoon demo, and nintendo sure is being cheeky about it, it's just that it's probably not going to happen...