Who say that it will be Half as long as resident evil 2?
Absolutely noone.
In fact, based on the little information we got from the trailer, it seems as if the devs are actually expanding on the original game's storyline in order to extend its length for the remake. For instance, Carlos will be exploring the RPD according to his S.T.A.R.S. office comment, which could mean that there will be a playable segment with Carlos for this part before he meets up with Jill. Also, the fact that there are so many non-infected people running on the streets could indicate that the remake's story starts much earlier than September 28. Remember, at the start of the original game Jill was almost the only civilian left alive.
It would make sense too. Jill being locked up in her apartment for several days always felt unrealistic to me. If anything, Jill would bust a door and get the hell out of there, she wouldn't be sitting inside while people (including her friends) are getting eaten alive outside.
The campaign will undoubtedly be much longer. The question is how much bigger Carlos's role will be? In the original game the part where you play as Carlos is quite short, so it will be interesting to see how they expand on his story. Who knows, maybe he will even get his OWN separate campaign that will interconnect with Jill's?
This is just a theory, but I believe that the last part of the trailer when a voice says "Commitment, honesty, integrity. These are the core values that create the foundation for Umbrella Corp" is part of a cutscene at the very beginning where someone high up in Umbrella is talking directly to Carlos and the other UBCS soldiers.