Quoting both, if the grain feed is sufficient there is no slop, I agree with you on commercial pellets and crumbles, just oatmeal consistency on water, any more is just slop.... I add extra water to my feed now (feed mill mash heavy on the grains, lots of powder also though) just for the extra barrier. I brew beer and make sauerkraut, lots of sauerkraut, wouldn't skimp on the barrier on them. With a good grain mix feeding the birds the water runs right off, no slop, and I continually use the 'water' for the next batch.
Water runs right off if you have a good grain mix.
Not sure what the "barrier" is for, exactly, but it's hugely not necessary in fermented feed. Things that stand around much longer, like kraut, is fine for keeping submerged so that one doesn't get that layer of scoby plus mold touching the kraut(wouldn't hurt it if it did, but still nice to not have it there), but FF is stirred and/or fed out daily, so no standing around. All that excess water and needing to strain it off and such is just a mess.