I think there is confusion going on between concentrate (real juice, evaporated to a syrup, lots of sugar) and extract (flavour only)
concentrate would referment unless you took steps to prevent that by stunning the yeast and keeping the beer cold, or by pastuerizing the bottles once the proper level of carbonation has been reached. it's going to take a lot of concentrat (1 quartish as the recipe says)
flavour extracts should be added to taste unless you are confident in the amounts you want to use. They taste nasty to me, even the good ones but that's an opinion thing.