So far, more and more evidence has been collected towards the achievements being real, which in-turn leads to MP being real. The announcement may not be in relation to MP but that doesn't rule out the existence of MP in any way.
I actually think with the evidence matching the achievements that were leaked. I would be very surprised if it was not mp.
Pretty sure their announcement is this:
Also, another demo at PAX?
A new trailer is not really a 'big announcement' unless mp gameplay is in the new trailer. The only other possibility is the warden is featured somewhere. Still, my money is on Mp.

and if it is mp then it's what will finalise my bargain bin decision.