The alkaline-acidic-balance-thing of the human body is still not vindicable from a scientific and a medical standpoint, and can't be influenced by nutrition.
your absolutely right, what you eat has zero efect on your bodies PH. your body has an internal regulation system that keeps your PH the same at all times, and if it did ever fluctuate, youd probably end up dead.
Now, that being said, certain food yeild a acidic content, while certain foods yeild an alkaline content. if you are eating more of one then the other, your body has to work to offset that inbalance. that process of ofsetting an imbalance is what you want to avoid. in order to offset a heavy acidic diet, your body has to take glutamine from muscle and calcium from bone. and it must utilize some catabolic hormones to do so.