I don't know how much of a "small" amount can cause problems, but there should be very little lactose left in yogurt. Lactobacillus turns that into lactic acid.
I'm not quite sure I understand the process of what they are doing here. Using bacteria to convert the lactose to galactose, which is still not fermentable by regular yeast, that I know of, so they must be using brett to create the alcohol. Either way, this is a lot of technology to throw at a waste product. I understand wanting to be resourceful and use things, but not when the end result will be drastically more expensive than a standard product. Who wants a 12$ pint of 3% beer? It should add value but not be over the top.