As I see it, the idea that they're 'lactose intolerant' is a bit of a trendy grab. Lactose intolerance in people leads to various issues like bloating, and this is where the term comes from, but there's no evidence that chickens suffer these effects unless someone feeds them pasteurised whole milk or a lot of powdered milk (in which case they do get diarrhea). Low lactose products should be fine.
Powdered skim milk has also been used as a treatment for cocci. There is good science for this, though it's not widely recommended (probably because it's only of limited benefit).
Farmers have always given lightly soured milk to poultry. Souring (e.g. kefir) removes a large majority of the lactose while providing probiotics.
It's fair enough to say high amounts of lactose from straight milk (or possibly yoghurt) can be harmful to birds, but one can find science to support many different approaches, including judicious soured milk feeding. Science works by excluding variables, but life has variables.
Everything is toxic at some level. If the benefits (e.g. probiotics, calcium) outweigh the negatives then (unlike a true poison) it has a place!
Edited to add:
By the way, apparently the probiotics in yoghurt don't survive to colonise the digestive tract. They can stop other unhealthy bugs from doing so (which is a good thing) but they tend to die off themselves. Have you thought of using kefir? Apparently kefir probiotics are much hardier and can colonise the gut (good for artificially brooded birds that aren't getting probiotics from mom)...
cheers
Erica