I'm not doubting or confirming what you are saying, only commenting. I think if your unclear as to why peoplle think this, you have only to look to our medical community. I am a mother of 4...ages 5yrs old to almost 20, and for almost 20 years now, when any of my children have respiratory issues or colds, etc...have always been told to limit or remove dairy consumption until it clears due to causing it to thicken mucus making harder to clear. So until recently I suppose, the medical community believed that also. I must be one of those who are mistaken in believing in the milk mucus phantom, because when I drink a big glass of milk, I would swear my saliva feels much thicker for a brief time until the milk is swallowed and cleared from my throat. I'm sure its not actually thicker saliva, but I'm also sure that when having congestion or cough, adding thick milk to my bacteria infected, thickening from infection, saliva is probably not helpful.
Milk contains sugar, at 11g per serving of 8 oz., that's almost 3 teaspoons of sugar, which activates insulin, which produces mucus, but not in everybody, of course. If anyone has any doubts, wait until you seem mucus-free, then drink a thick milkshake.
I know from experience, I am allergic to dust, and a sensitivity to dairy, when I drink milk, I have chronic problems with mucus, Asthma, and constant infections to the ears and throat. Over the last 8 years, I have stopped drinking Dairy, and my allergies are less, and I don't have asthma attacks. When I drink milk the fat in the milk covers and coats my tongue, and throat, When I get a cold I make a honey lemon tea, so that the honey coats my tongue removing irritation, the lemon is to help my immune system, tea for me is to change the taste. I hate the honey taste, but it is a proven remedy to sooth the throat. Since I stop drinking milk every thing tastes better, I have a great sense of smell, and I no longer feel bloated, and the feminine pain is less, My irritable bail has lessen unless I eat aged cheese.. then i get really bad motions. All because Of allergies. Not to mention, if I walk into to a dust filled house, school, etc, my nose runs clear liquid, If I go out side it stops. Since I have been drinking Soy milk all my symptoms have lessened. I am no longer asthmatic. But adding milk to the problem only aggravates the problems already there. It doesn't cause it, but it makes it worse.