I'm not sure it absorbs water, but it definitely isn't good as the only base coat. I had a '87 s-10 that was flat black, prepped and sanded no clear coat, and the paint stuck fine, but it grabbed dust and got dirt stains like the devil (maybe it DID absorb water). are you putting a clear over it? or just the flat? (I kinda like the grey primer look on the new impalas that looks cleared) that would be ok, since the clear would protect it.
I know with house paint, gloss paint is a "harder" finish, and is easier to clean/less stuff sticks. flat is kinda only good for ceilings since its hard to clean and easy to stain. I never hear anything about "absorbtion" I understood it to be more of flat having a non-smooth surface (lots of little crevaces) and gloss was self smoothing if you will (hard smooth surface)
hope all this rambling made sense... good luck!