Well this is interesting, but I wouldn't write off it being a Nex-Box launch title yet. First off Microsoft and other manufacturers and publishers have lied before about their products. So this confirmation really doesn't mean all that much, there are still over whelming evidences to support the fact it could be a Nex-Box launch title.
Why haven't any trailers been shown, other then the teaser and the IGN video? Why no screenshots? Why does Microsoft routinely avoid questions about it being on 360? Other then this statement Microsoft has rarely mentioned it as a 360 title, at PAX I talked with 343 members and asked if it was a 360 or Nex-Box title and they said they couldn't tell me.
Also in 2008, 343 began hiring developers to work on a Next Generation Halo title. In the job description Microsoft wanted someone familiar with the latest PC engines and could work on new hardware. So the question is why did 343 begin hiring staff to work on a next generation Halo title back in 2008 if Halo 4 isn't a next generation title? Are you telling me that 343 has spent over 4 years developing Halo 5? By the time Halo 5 is released say 2014 it will have been in development 6 years. Which makes Halo 4 being a 360 title iffy, I mean if you add up the time line job statements and rumors it paints a very clear picture that Halo 4 could be a Nex-Box title.
So if it is indeed a 360 game they have gone out of their way to leave suspicion.
Another possibility is a multiplatform release like Twilight Princess, Halo 4 could be released on both Nex-Box and 360 thus being a 360 title. It could also be released on a single disk playable in one resolution (Version) for 360 but when played on Nex-Box it could be an improved version.
I won't believe anything Microsoft says until E3 or when ever they announce Nex-Box. Simply because until they show their cards their is just to much evidence that suggests Halo 4 could be a launch title. I note that Microsoft still hasn't shown any definitive proof to actually prove Halo 4 is a 360 title.
Microsoft's word is not worth the paper its written on, just like Nintendo and Sony things are subject to change without notice. If they were ever intended to be the original way at all!