Realistically outside of Chief's new armour, there wouldn't be much to explain. Forerunner stuff looks like more modern Forerunner stuff which could cause some confusion because it isn't in line with what we saw on the Alpha and Delta Halos, however, those two rings also had noticeable differences .
The hog is the H4 model which appears to be the new mainline canon design for the hog, considering its use in H2A.
The Battle Rifles are just the H4/5 ones.
The reach style marine gear in black could also be a point of confusion alongside Chief, but when you consider this game will likely be taking place around May 2560 it's not hard to believe that the UNSC forces linked with what I assume to be infinity have had there resources stretched over the last year and a half, forcing them to use older tech. Even if said old tech is the gross Reach gear, for the marines at least.
I imagine that we will get an explanation, the real question is if it follows the MK VI situation where the nanobots explanation was given through interviews and not the actual game.
All that being said the "Trailer" is nothing more then tech demo to show off the engine so I would imagine everything is subject to change.
Judging by the "trailer" and the fact that the current art director for Halo is one of the people form HW2 that is likely the case.