IIRC, they weren't going to release a demo for it at all, but after the PC Gamer community had a moan, they decided that it might be within their best interest to oblige.
I can only go by the feedback I've been reading about this demo, along with the screenshots as I still haven't been able to get past the account creation process. And I'm no expert of course, but due to time constraints and lack of resources a not so illogical hypothesis is that this demo for the "PC" is basically something they threw together/ported with the 360 code. To further back this up: a lot of people are saying that it's running in DX9, which goes hand in hand with the fact that the installer is only 1.6GB.
However, I played the single player Alpha code in January at an EA gaming event in Sydney, and it didn't "feel" like a port then. So I still live in hope that ONLY this demo has been consolified, but not the FULL release for the PC.