They are all filled. They state that they cannot offer you funding. But, sometimes, professors get grants and a position opens up. They are basically saying there isn't anything for you now, but IF something comes available at a later time MAYBE you can be offered support. If funding is an issue for you, you will certainly want to keep other university options open.
It sounds like you will only have two quarters of support. There are three different kinds (GSR, TA and Readership). They each pay different amounts. But they don't tell you which one you will get, just that you are guaranteed some kind of support for 2 quarters. You may not find out until quarterly assignments are made. I'd call and get more details.