I grew up on a dairy farm and we gave the test milk to the cats all of the time with no apparent ill effects.
That was what I thought too.
I thought the idea sounded daft so I put out a saucer of milk.
Then I spent several hours cleaning up cat puke from beds, chairs, carpets.
I wonder if the difference is that you fed them fresh milk wheras the stuff I gave to mine was pasteurized?