I was referring to the area between the deck and the "curb" as the "channel". The curb would not only control the grass, but keep the fill material in place, and as you say, keep the grass and rock from mixing.
I'm not sure what "decomposted granite fill" is exactly. If it has "fines" in it, it will probably pack pretty hard and tight and water would not easily go through it. This would be true even if you do not initially pack it so tight. It will compact on it's own over time. It also depends on your underlying soil. I'm in Florida, so it's sand. If I pour a 5 gallon bucket of water anywhere, it's gone in an instant. If your soil drains pretty quickly, a gravel/granite product without fines would pack fairly tight (if it has sharp/irregular edges) and still let the water through, though not as much water as a round material like pea gravel or a small river rock. It would be nice if you could get a small amount and try it in an area. What you put on top of it is irrelevant, except for looking nice. It's how the thing is going to drain when your done.
If the grass can take the water, because your soil drains fast and doesn't make a bunch of mud, a tightly packed, almost impervious material would be OK, it'll just shuttle a majority of the water to the grass.
You may get heavy rains where you are. The "drain" may be to keep the runoff from the deck from getting directly to the grass and really making it wet and muddy with even slight traffic. I suppose this would be worse with slow draining soil that is loamy (makes mud easily).