Posters say that something helped them - I don't dismiss that, or any of your other precautions. It is worth trying, it may help someone else - or it may not. Websites where someone is pushing a pill or potion which will help should be treated with caution. Those who post on them are those who are enthusiastic, not those who found it didn't work and abandoned it - and the site owners are keen for you to buy.
What I do disagree with is your statement "I dont get why there has to be good evidence for stuff". Anyone anywhere, with no medical knowledge whatsoever, can tell their friends "I always stand on my head for exactly three minutes every morning, and I never ever eat mushrooms. I have not had a cold for ten years! So if you want to be cold-free - stop buying mushrooms and spend three minutes upside down." That is anecdotal evidence - "It worked for me, so it'll work for you!"
Good evidence would be a thousand or more people, picked at random from the general population, of all ages, sexes and backgrounds. Divide them into two groups and feed one half on mushroom casseroles, bacon and mushrooms, chicken and mushroom pie, mushroom stroganov and so on. Don't give the other half a single mushroom.
Get the mushroom group to stand on their heads for three minutes before breakfast each day (not just after eating the mushroom casserole, that could have unfortunate side effects)
Record their health regularly. At the end of the trial (which should take place in the season when viruses are prevalent) see how many of each group caught a cold or flu.
That is evidence-based medicine.
Elderly fellow gran has become loudly racist