Kumis has an alcohol content of less than 1% on average. In modern times, you're more likely to accidentally swallow a bit of mouthwash every day which has a significantly higher alcohol content of over 10%, even higher than most beers. Not all booze is created equal...but to medieval-fantasy people, all booze was just booze.
That being said, the health benefits of kumis was more likely from the probiotic microbes good for your gut flora, as well as other milk-nutrients like vitamins and minerals in place of breast-feeding, rather than the alcohol itself in it. That, and the fact that the quality of water itself wasn't always boiled before drunk, as water sanitation wasn't understood well then...Which was why in many instances in the past, and even in some places to this day, drinking booze even with a low-alcohol content would be the "cleaner" option than drinking local water, as it was more likely to have been purified during the fermentation or distilled afterwards.