I have not found this to be true at all. We have a whole scad of beautiful lilies in our garden, which are toxic to chickens, and they don't touch them at all. They do huddle under them for shade, and scratch under them for bugs. The lilies love all this attention and cultivating, not to mention the fertilizing, and are thriving. They have been sharing the space for about five years with no casualties on either side..
Our chickens also free range part of the day. We have a mad assortment of other bulb flowers, including daffodils and irises and many more lilies and the chickens do not bother them at all. It seems to me that chickens must naturally avoid toxic plants in nature, as must all birds and wild things. Otherwise they would soon become extinct.