Not sure I have ever seen a fluid saddle. plenty of gel saddles but not fluid. For a saddle you can have padding but you want it support your ischial tuberosity (sit bones) and not just sink in. If they aren't being well supported and you have to compress a lot of material in order to get supported you are creating pressure points which can cause pain.

My guess is if the saddle is fluid you would hope it is not like a water bed or something I don't think that would be very good down there. The Trek website doesn't really tell you what the fluid is or how it is contained or anything. Also that saddle is for a very upright bike so if you are not upright on your bike the saddle is probably going to be way too wide.