Miuramir.... In that quote you took from my post I very specifically said that fruits are an exception. Yes, apples (which are a fruit, I might add) have all the water they need. However, many things in DF that are brewable do NOT have enough water... namely the grains. Rye, Oats, Wheat, Milet, etc would all require additional water to brew. Also, as GavJ mentioned, mushrooms simply lack the sugar content to be brewed. Fermented sugar leads to ethanol (what gets you drunk), but the only "sugar" in mushrooms is bound up in tough structural fibers (their version of cellulose). While it is theoretically possible to brew a beverage out of them, you would need excessive amounts of mushrooms and far more energy than its worth to break down the chitin (to get at the glucose) is just not worth it.
Also, as GavJ mentioned, the underlying ecology of underground plants is just groundless (sorry, had to >.>) For plants to produce sugar, they need an energy source. Typically, this is the sun. Plants take in sunlight to bind other elements together and create glucose. Fungus will leech nutrients out of other organic matter- but where would that other organic matter get its nutrients? Are we to believe the thriving underground ecosystem is piggybacking on the organic matter (ie nutrients) that stumbles down into it from the surface? It would simply not be enough. There must be some sort of alternative energy source serving as the foundation. The only solution I can remotely think of is thermo/chemophilic bacteria. However, these types of bacteria typically thrive in environments that would kill most anything else on earth. Therefore for this to work, there would need to be an intermediary that can eat the thermo-philes, but other creatures can then in turn eat. Bacteria that live 1 mile down into the earth's crust have also been found feeding on background radiation, but these were -very- few in number. They could barely eek out a living, yet alone provide a backbone for an entire food web. Another problem, is that mineral/chemical eating bacteria would naturally pass their food source up the chain. Something that eats arsenic would likely kill anything that tries to eat it.
ofc, the most obvious answer would be to just say "something something MAGIC!" but DF aspires to be as realistic as it can. However, as has been mentioned, trying to make booze-making more realistic while ignoring how broken farming is seems silly. Its not just that the underground plants grow without lack of nutrients, but ALL plants grow thusly. Even above ground, there is no such thing as "soil depletion" You can pull thousands of crops out of DESERT SAND without ever having to either irrigate or use fertilizer. Several days ago I actually put up a suggestion to add compost heaps to the game. Ideally you'd throw in all the food scraps (mostly butchering remains, but any spoiled food and vermin corpses) with fallen leaves/flowers (which serve no purpose) and wait a few months for them to turn into compost. You'd then need to mix this compost into the soil to keep growing crops in the same spot (or underground).