Hi,
I occasionally make my own yoghurt and I've never heard this before. I did a quick google it seems kefir and koumiss yoghurt may naturally contain a very small amount of alcohol. I think if your making run of the mill natural or Greek yoghurt it should be fine. Also it is very difficult to completely avoid trace amounts of alcohol since its occurs naturally in fruit juices, etc. so I wouldn't worry if it were me (I am also unlikely to say no to a slice of cake that may contain vanilla extract - although I probably should!)
I love homemade yoghurt!
x