Its true that milky water during tank startup is almost always due to either small particles from the substrate or from a bacterial bloom. And it sounds like the OP (clos) has come to the right conclusion that the cloudiness is nothing to worry about.
A couple of details though: Bacterial blooms are extremely common in newly setup tanks and they can happen quite early in the process, even before ammonia is added. This is because what happens is that the conditioner we add to dechlorinate/dechloraminate frees up water for bacteria to multiply. Often the water will have many "organics" in it (tiny carbon based molecules.) These are just present, in varying amounts, in different water supplies. Heterotrophic bacteria (not the autotrophs we want to grow in our filters) that are free-swimming and fast mulitplying will suddenly find themselves with this large organic food supply and no chlorine to inhibit their reproduction and they will "bloom" by the millions, becoming visible as a milky whiteness in the water. So the ammonia is not really part of the picture as its not their food and the timing is basically post-conditioner, in newly filled tanks.
Again, its all a harmless curiosity. The heterotrophs will make some biofilms that will look like gray to white-ish muck on various interior surfaces of the tank, but eventually this initial supply of organic molecules will be eaten up and the bloom will die off and the water will go clear again. And by the end of fishless cycling much later it will definately be gone.
~~waterdrop~~
