Cabbage does contain small amounts of a substance that is not good for chickens or humans. A whole lot of foods contain something that is not good for chickens or humans. There was a recent story about a woman who ate a few pounds of Bok Choi every day and her thyroid eventually shut down. I hate to think what that much roughage did to her digestive system.
If you feed them massive amounts of anything, including cabbage and things related to cabbage, then they could have problems. If you feed them moderate amounts of treats, their bodies will process the "poisons" and eliminate them, causing no harm. If you eat massive amounts of anything, then you could be causing harm to yourself. I remember a story a few years back where a woman drank enough water to kill herself as part of a radio contest to see how much water she could drink before she went to the bathroom.