I’m happy to help clear up the argument for you. First, I have to say, “good on you” (as my New Zealand harvest interns used to say) for questioning what you hear; discussing vinous conundrums is how we learn! It’s so engaging to have these discussions with our winemaking peers, opening our minds to different views and ways of thinking; it happens in professional circles all the time!
To answer the first question: Indeed, you are correct. Adding acid to a KMBS solution makes it more effective as a sanitizing agent. This is because the lower the pH, the higher the percentage of the two “biologically active” forms of sulfur dioxide — molecular SO2 and bisulfite (HSO3–) — you’ll have in your aqueous solution. These are the two forms that directly disrupt biological activity (which is what we’re trying to stop with sanitation in the first place) of spoilage yeast and bacteria. The higher the pH (meaning the lower the acidity), the higher concentration of KMBS powder you’d need to do the same job. Basically, the acid you’re adding forces more of the sulfur dioxide to exist in a more potent form.
To answer your second question, my response is very simple: Cost. Tartaric acid is many times more expensive than citric acid, which is why, since it’ll just end up getting sprayed around the cellar and eventually going down the drain, most wineries of any size don’t use tartaric acid in sanitizing solutions and sprays. However, as you know, home winemakers aren’t often as constrained by cost as large commercial establishments, so if you’ve got a bag of tartaric sitting around and you’d like to use it, go right ahead. Citric acid metabolism during malolactic fermentation is an uncommon (but significant) risk (causes carbon dioxide and some metabolic byproducts), but most wineries don’t worry too much about it. I’d wager that the amount of citric left sitting undissociated on dry equipment surfaces post-sanitation is small.
