They've planned for years to make another trilogy. It's not something they only recently decided to do. It's been the plan since before Halo 3 even came out. Remaking Halo 1, though, is selling out and completely unnecessary. Plus they're only doing 7 maps, which is silly, and Halo 1 is best for the PC with a poor setup that results in leads. Halo is as good as it is because of design flaws.
Also, he wasn't in cryo either at the end. There's no reason to assume that this is immediately following the events of Halo 3's ending. There most likely was some amount of time period between it. Plus it's just a trailer and nothing more than that in any case, so these details don't matter really.